Major README update

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

README.md

@@ -9,4 +9,49 @@ that makes it easier to deploy it in production, including the initial setup ste
  [docker]: https://www.docker.com/
  [writefreely]: https://github.com/writeas/writefreely
 
-There will be more documentation soon...
+## Overview
+
+The image is set up to use SQLite for the database, it does not support MySQL
+out of the box - but you can always provide your own `config.ini`. The config
+file, the database, and the generated keys are all stored on the single volume
+the image uses, mounted on `/data`.
+
+The primary purpose of the image is to provide a single-step setup and upgrade
+experience, where the initial setup and any upgrades are handled by the image
+itself. As such, the image will create a default `config.ini` unless one already
+exists, with reasonable defaults. It will also run database migrations, and save
+a backup before doing so (which it will delete, if no migrations were
+necessary).
+
+## Getting started
+
+To get started, the easiest way to test it out is running the following command:
+
+```shell
+docker run -p 8080:8080 -it --rm -v /some/path/to/data:/data \
+       algernon/writefreely
+```
+
+Then point your browser to `http://localhost:8080`, and you should see
+WriteFreely up and running.
+
+## Setup
+
+The image will perform an initial setup, unless the supplied volume already
+contains a `config.ini`. Settings can be tweaked via environment variables, of
+which you can find a list below. Do note that these environment variables are
+*only* used for the initial setup as of this writing! If a configuration file
+already exists, the environment variables will be blissfully ignored.
+
+### Environment variables
+
+- `WRITEFREELY_BIND_HOST` and `WRITEFREELY_BIND_PORT` determine the host and port WriteFreely will bind to. Defaults to `0.0.0.0` and `8080`, respectively.
+- `WRITEFREELY_SITE_NAME` is the site title one wants. Defaults to "A Writefreely blog".
+- `WRITEFREELY_SINGLE_USER`, `WRITEFREELY_OPEN_REGISTRATION`,
+  `WRITEFREELY_MIN_USERNAME_LEN`, `WRITEFREELY_MAX_BLOG`,
+  `WRITEFREELY_FEDERATION`, `WRITEFREELY_PUBLIC_STATS`, `WRITEFREELY_PRIVATE`,
+  `WRITEFREELY_LOCAL_TIMELINE`, and `WRITEFREELY_USER_INVITES` all correspond to
+  the similarly named `config.ini` settings. See the [WriteFreely docs][wf:docs]
+  for more information about them.
+
+ [wf:docs]: https://writefreely.org/docs/latest/admin/config