Add documentation

Author: fritterhoff

docs/optional/README.md

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+# Optional Gitlab and MISP Container
+
+You can host your own gitlab and MISP instance using the `docker-compose.optional.yml` and the `misp-docker` submodule.
+
+Please note, that both services start completly unconfigured and must be setup manually afterwards. This also includes setting up user accounts, access tokens or kroki.
+
+## Usage
+
+### Configuration
+
+You must configure several options (e.g. the mysql credentials for misp) in your env file.
+
+### Starting gitlab
+
+You can start the gitlab server either by using `make gitlab` or by running a `docker-compose -f docker-compose.optional.yml up gitlab`. 
+
+### Starting MISP
+
+You can start the gitlab server either by using `make misp` or by running a `docker-compose -f docker-compose.optional.yml up misp`. Please note, that starting/building MISP the first time may take up to 15 minutes (depending on your infrastructure).