docs: updated README

plugins/extractors/http/README.md

@@ -13,7 +13,8 @@ source:
       method: "GET"
       headers:
         "API-TOKEN": "XXXXX"
-        "Content-Type": "application/json"
+      content_type: application/json
+      accept: application/json
     response:
       root: "data"
       mapping:
@@ -26,7 +27,7 @@ source:
           fullname: "name.fullname"
           status: "terminated"
           attributes:
-            business_title: "business_title"
+            business_title: "CONST.employee"
             company_hierarchy: "company_hierarchy"
 ```
 
@@ -37,6 +38,8 @@ source:
 | `request.url` | `string` | `https://odpf.io` | HTTP service URL | *required* |
 | `request.path` | `string` | `employee/list` | Path to the service | *not required* |
 | `request.method` | `string` | `GET` | HTTP request type | *required* |
+| `request.content_type` | `string` | `application/json` | Request body content type | *not required, default: application/json* |
+| `request.accept` | `string` | `application/json` | Expected response's content type | *not required, default: application/json* |
 | `request.headers` | `key-value` | `"API-TOKEN": "XXXXX"` | Headers in to form of key value pairs. Supports multiple values seperated by `,` | *not required* |
 | `response.root` | `string` | `data` | `.` seperated [json path](https://jsonpath.com/) representation to the key containing data | *required* |
 | `response.mapping` | `nested key-value` | `-` | Defines how the data is to be mapped with the asset model. Uses json path representation wrt `root` | *-* |
@@ -46,6 +49,11 @@ source:
 | `response.mapping.type` | `string` | `type: "user"` | One of `table, topic, dashboard, user, bucket, job` | *required* |
 | `response.mapping.data` | `nested key-value` | `-` | Nested key-value based on the definition of asset type  | *required* |
 
+### Note: 
+
+To send constant values of fields in mapping, use `CONST` seperated by `.` and then the value.
+For instance, `email: CONST.user@odpf.com`
+
 ## Outputs
 
 Output can be any of the asset type.