First attempt at some documentation

docs/installation.md

@@ -0,0 +1,39 @@
+# Installation
+
+## Manual
+
+```sh
+cargo install --locked --git https://github.com/lennart-k/rustical
+```
+
+## Docker
+
+```sh
+docker run \
+  -p 4000:4000 \
+  -v YOUR_DATA_DIR:/var/lib/rustical/ \
+  -v YOUR_CONFIG_TOML:/etc/rustical/config.toml \
+  -v YOUR_PRINCIPALS_TOML:/etc/rustical/principals.toml \
+  ghcr.io/lennart-k/rustical
+```
+
+## Configuration
+
+### TOML
+
+### Environment variables
+
+The options in `config.toml` can also be configured using environment variables. Names translate the following:
+
+```toml title="config.toml"
+[data_store.toml]
+path = "asd"
+```
+
+becomes `RUSTICAL_DATA_STORE__TOML__PATH`.
+Every variable is
+
+- uppercase
+- prefixed by `RUSTICAL_`
+- Dots become `__`
+- Arrays are JSON-encoded