add documentation

doc.md

+# JastAdd in OpenAPI Parser
+
+This is a documentation of differences between the document of [OpenAPI Specification in version 3.0.2](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md) and [OpenAPI parser for Java projects](https://github.com/openapi4j/openapi4j).
+
+## Differences
+
+Object Name | Description
+---|:---:
+[OpenAPI Object](#openAPI)|variable "path" is defined differently ( in spec. as one object and in parser as map )
+[Paths Object](#paths)|see OpenAPI Object
+[Path Item Object](#pathItem)|variables in type "Operation Object" are defined the part of a map in openapi parser
+[Operation Object](#operation)|Responses Object is defined as a map in openapi parser
+[Parameter Object](#parameter)|allowEmptyValue does not exist in openapi parser
+[Encoding Object](#encoding)|variable "allowReserved" does not exist in openapi parser
+[Responses Object](#responses)|see Operation Object
+[Callback Object](#callback)|openapi parser has expressions as a map
+[Link Object](#link)|openapi parser has variable "headers" instead of variable "requestBody"
+[Reference Object](#reference)|no Reference Object in openapi parser
+[OAuth Flow Object](#oAuthFlow)|openapi parser has new variable "configuration"
+
+## Contents
+
+### <a name="openAPI"></a>OpenAPI Object
+
+- variable "path" is defined differently ( in spec. as one object and in parser as map )
+- Links to compare
+  - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#oasObject)
+  - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/OpenApi3.java)  
+
+
+### Info Object
+
+- no differences
+
+### Contact Object
+
+- no differences
+
+### License Object
+
+- no differences
+
+### Server Object
+
+- no differences
+
+### Server Variable Object
+
+- no differences
+
+### Components Object
+
+- no differences
+
+### <a name="paths"></a>Paths Object
+
+- see OpenAPI Object
+
+### <a name="pathItem"></a>Path Item Object
+
+- variables in type "Operation Object" are defined the part of a map in openapi parser
+- Links to compare
+  - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#pathItemObject)
+  - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/Path.java)
+
+### <a name="operation"></a>Operation Object
+
+- Responses Object is defined as a map in openapi parser
+- Links to compare
+    - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#operationObject)
+    - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/Operation.java)
+    
+### External Documentation Object
+
+- no differences
+
+### <a name="parameter"></a>Parameter Object
+
+- allowEmptyValue does not exist in openapi parser
+- Links to compare
+    - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#parameterObject)
+    - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/Parameter.java)
+
+### Request Body Object
+
+- no differences
+
+### Media Type Object
+
+- no differences
+
+### <a name="encoding"></a>Encoding Object
+
+- variable "allowReserved" does not exist in openapi parser
+- Links to compare
+    - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#encodingObject)
+    - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/EncodingProperty.java)
+
+### Responses Object
+
+- see Operation Object
+
+### Response Object
+
+- no differences
+
+### <a name="callback"></a>Callback Object
+
+- openapi parser has expressions as a map
+- Links to compare
+    - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#callbackObject)
+    - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/Callback.java)
+
+### Example Object
+
+- no differences
+
+### <a name="link"></a>Link Object
+
+- openapi parser has variable "headers" instead of variable "requestBody"
+- Links to compare
+    - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#linkObject)
+    - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/Link.java)
+
+### Header Object
+
+- no differences
+
+### Tag Object
+
+- no differences
+
+### Reference Object
+
+- no Reference Object in openapi parser
+- most objects has variable "Ref" and method getRef() or isRef()
+
+### Schema Object
+
+- not relevant in current JastAPI
+
+### Discriminator Object
+
+- not relevant in current JastAPI
+
+### XML Object
+
+- not relevant in current JastAPI
+
+### Security Scheme Object
+
+- no differences
+
+### OAuth Flows Object
+
+- no differences
+
+### <a name="oAuthFlow"></a>OAuth Flow Object
+
+- openapi parser has new variable "configuration"
+- Links to compare
+    - [In OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.2.md#oauthFlowObject)
+    - [In openapi parser](https://github.com/openapi4j/openapi4j/blob/master/openapi-parser/src/main/java/org/openapi4j/parser/model/v3/OAuthFlow.java)
+
+### Security Requirement Object
+
+- no differences
+