doc: write initial readme

README.md

@@ -1,2 +1,50 @@
 # mantisbt-docker
 Docker image for Mantis Bug Tracker https://www.mantisbt.org/
+
+# Why this image?
+
+There are some other alternative images exist already such as [vimagick/mantisbt](https://hub.docker.com/r/vimagick/mantisbt/) or [mikroways/mantisbt](https://hub.docker.com/r/mikroways/mantisbt/). Why do we need yet another image?
+
+Answer is - those images don't have all the nice features :)
+
+- Always latest MantisBT version.
+- Comes with the latest PHP version (7.4 as for today)
+- Allows to easily configure presence of `admin` service folder
+- Comes with built-in integration with Gitlab and Github [source plugins](https://github.com/mantisbt-plugins/source-integration)
+- Example `docker-compose.yml` file provided for getting started in one click!
+
+
+# Quick start
+
+- Download `docker-compose`
+- Check the environment variables (at least you need to set MASTER_SALT env variable, [doc](
+https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.config.security))
+- `docker-compose up -d`
+- Open browser at `localhost:8989/admin/install.php` and follow installation instructions (TODO: DB credentiasl)
+-- Ignore `Config File Exists but Database does not` warning and proceed installation
+- Log in as `administrator`/`root` (default credentials) and confugre whatever you need (typically you want to create your own Admin user and disable built-in "administrator" first)
+- Check MantisBT own's checks at `localhost:8989/admin/`. Note: several warnings are expected to be "WARN" due to issues in MantisBT, such as magic quotes warning ([#26964](https://www.mantisbt.org/bugs/view.php?id=26964) and "folder outside of web root" warnings ([#21584](https://mantisbt.org/bugs/view.php?id=21584)))
+- When ready to move to production, either remove `MANTIS_ENABLE_ADMIN` env variable or set it to 0 - this will remove "admin" folder from the installation.
+
+For further details refer to [official documentation](https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.install.new)
+
+
+# Extensions
+
+## Custom config settings
+
+If you need to customize more options in config, create `config_inc_addon.php` file and mount it to `/var/www/html/config/config_inc_addon.php` in container. This fill will be added to default `config_inc.php`. Mounting it will allow you to see the changes instantly without rebuilding/restarting the container.
+
+## Email
+
+There are following env variables supported:
+
+- EMAIL_WEBMASTER - maps to `g_webmaster_email`
+- EMAIL_FROM - maps to `g_from_email`
+- EMAIL_RETURN_PATH - maps to `g_return_path_email`
+
+More details are available in [documentation section](https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.config.email)
+
+## Custom plugins
+
+In order to add your own custom plugins into the image, either create your own Dockerfile and copy extra plugins to `/var/www/html/plugins/` or add volume in docker-compose to mount extra plugin directly inside existing image `./custom_plugin/:/var/www/html/plugins/custom_plugin/`