feat: complete README

https://gravitee.atlassian.net/browse/APIM-3985

README.adoc

@@ -1,17 +1,54 @@
-= Gravitee Policy mTLS
+= mTLS policy
 
-== Phases
+ifdef::env-github[]
+image:https://img.shields.io/static/v1?label=Available%20at&message=Gravitee.io&color=1EC9D2["Gravitee.io", link="https://download.gravitee.io/#graviteeio-apim/plugins/policies/gravitee-policy-mtls/"]
+image:https://img.shields.io/badge/License-Apache%202.0-blue.svg["License", link="https://github.com/gravitee-io/gravitee-policy-mtls/blob/master/LICENSE.txt"]
+image:https://img.shields.io/badge/semantic--release-conventional%20commits-e10079?logo=semantic-release["Releases", link="https://github.com/gravitee-io/gravitee-policy-mtls/releases"]
+image:https://circleci.com/gh/gravitee-io/gravitee-policy-mtls.svg?style=svg["CircleCI", link="https://circleci.com/gh/gravitee-io/gravitee-policy-mtls"]
+endif::[]
 
-[cols="4*", options="header"]
+== Phase
+
+[cols="2*", options="header"]
 |===
 ^|onRequest
 ^|onResponse
-^|onMessageRequest
-^|onMessageResponse
 
 ^.^| X
 ^.^|
-^.^|
-^.^|
+
+|===
+
+== Description
+
+You can use the `mtls` policy to verify a client certificate exists as part of the request.
+
+This policy does not ensure that certificates are valid, since it is done directly by the server.
+
+== Compatibility with APIM
+
+|===
+| Plugin version | APIM version
+| 1.x            | 4.5 to latest
+|===
+
+== Errors
+You can use the response template feature to override the default response provided by the policy. These templates must be defined at the API level (see the API Console *Response Templates*
+option in the API *Entrypoints > Response Templates* menu).
+
+The error keys sent by this policy are as follows:
+
+[cols="2*", options="header"]
+|===
+^|Key
+^|Parameters
+
+.^|CLIENT_CERTIFICATE_MISSING
+^.^|-
+.^|CLIENT_CERTIFICATE_INVALID
+^.^|-
+.^|SSL_SESSION_REQUIRED
+^.^|-
+
 |===