docs: add installation instructions

INSTALL.rst

@@ -1,2 +1,74 @@
 Installation
 ============
+
+Install the package into the same environment as your Invenio instance:
+
+.. code-block:: console
+
+   pip install invenio-github
+
+
+In order to use it, you need to create a GitHub OAuth App. Follow the instructions
+`here <https://docs.github.com/en/developers/apps/building-oauth-apps/creating-an-oauth-app>`_.
+During the creation, note down the `client ID` and generate a `client secret`.
+
+Now you need to create a random string to be used as `webhook secret`. You can use the following command to generate a random string:
+
+.. code-block:: console
+
+   openssl rand -hex 32
+
+Then, you need to set the following variables in your Invenio configuration (`invenio.cfg`) in order to enable the GitHub OAuth client:
+
+
+.. code-block:: python
+
+   # Invenio-OAuthclient
+   # -------------------
+
+   OAUTHCLIENT_REMOTE_APPS = {
+       "github": github_remote_app,
+   }
+
+   OAUTHCLIENT_REST_REMOTE_APPS = {
+       "github": github_remote_app,
+   }
+
+   GITHUB_APP_CREDENTIALS = dict(
+       consumer_key=<your_client_id>,
+       consumer_secret=<your_client_secret>,
+   )
+
+    # Invenio-GitHub
+    # ==============
+    GITHUB_INTEGRATION_ENABLED = True
+    GITHUB_WEBHOOK_RECEIVER_ID = "github"
+    GITHUB_WEBHOOK_RECEIVER_URL = SITE_API_URL + "/receivers/github/events/?access_token={token}"
+    GITHUB_SHARED_SECRET = <your_webhook_secret>
+
+Make sure to replace `<your_client_id>`, `<your_client_secret>`, and `<your_webhook_secret>` with the actual values.
+
+Local Test Instance
+-------------------
+
+If you are running a local test instance, you can use smee.io to forward GitHub webhooks to your local
+instance. You can set it up as follows:
+
+1. Go to <https://smee.io/> and create a new channel.
+2. Set the `GITHUB_WEBHOOK_RECEIVER_URL` to the smee.io channel URL.
+
+   .. code-block:: python
+
+      GITHUB_WEBHOOK_RECEIVER_URL = "https://smee.io/<your_channel_id>/?access_token={token}"
+
+3. Install the smee client:
+
+   .. code-block:: console
+
+      npm install -g smee-client
+
+4. Run the smee client to forward the webhooks to your local instance:
+
+   .. code-block:: console
+
+      npx smee-client --url https://smee.io/<your_channel_id> --port 5000 --path /api/receivers/github/events/