Describe why apikey is needed, where to get them and the format.

Fixes #319
Fixes #93 completely

README.rst

@@ -319,6 +319,23 @@ The manual override will be reflected in the output, as follows:
     (5/5) checklist
           ✓ skipped (reason: I'm using the Codacy dashboard to guide my development)
 
+Rate limit
+^^^^^^^^^^
+
+By default ``howfairis`` uses anonymous requests to the API of the source code platforms.
+However when a lot of repositories are checked you will exceed the rate limit of those APIs and checks will fail.
+To increase the rate limit you need to use authenticated requests.
+Your username and token can be passed to ``howfairis`` using environment variables called ``APIKEY_GITHUB`` and ``APIKEY_GITLAB``.
+The format of the environment variable values are:
+
+.. code-block:: shell
+
+  export APIKEY_GITHUB=<user who made the token>:<personal access token>
+  export APIKEY_GITLAB=<user who made the token>:<personal access token>
+
+Your personal access token should be created on `GitHub <https://github.com/settings/tokens>`_ and `GitLab <https://gitlab.com/-/profile/personal_access_tokens>`_ settings pages.
+No scopes have to be selected, being authenticated is enough to get higher rate limit.
+
 Contributing
 ------------