NPA-4284: Add small description about our request header policy

Author: ellie-bound1-NHSD

specification/validated-relationships-service-api.yaml

@@ -87,6 +87,9 @@ info:
     |[NHS login - combined authentication and authorisation](https://digital.nhs.uk/developer/guides-and-documentation/security-and-authorisation/user-restricted-restful-apis-nhs-login-combined-authentication-and-authorisation) |OAuth 2.0 authorisation code with API key and secret |No need to integrate and onboard separately with NHS login.  |No access to user information.                           |
     |[NHS login - separate authentication and authorisation](https://digital.nhs.uk/developer/guides-and-documentation/security-and-authorisation/user-restricted-restful-apis-nhs-login-separate-authentication-and-authorisation) |OAuth 2.0 token exchange with signed JWT             |Gives access to user information.                            |Need to integrate and onboard separately with NHS login. |
 
+    ## Headers
+    This API is case-insensitive when processing request headers, meaning it will accept headers regardless of the letter casing used. (e.g. X-Request-Id, x-request-id are treated the same). When sending headers back in the response, we preserve the exact casing as received in the original request.
+    
     ## Errors
     We use standard HTTP status codes to show whether an API request succeeded or not. They are usually in the range: