Add docs for environment variables

docs/SUMMARY.md

@@ -7,6 +7,7 @@
     - [Advanced](advanced.md)
     - [Settings](settings.md)
     - [Shell completion](shell_completion.md)
+    - [Environment variables](environment_variables.md)
     - [TUI](tui.md)
 - [Contributing](contributing.md)
     - [Documentation](documentation.md)

docs/environment_variables.md

@@ -0,0 +1,19 @@
+# Environment variables
+
+> Introduced in toot v0.40.0
+
+Toot allows setting defaults for parameters via environment variables.
+
+Environment variables should be named `TOOT_<COMMAND_NAME>_<OPTION_NAME>`.
+
+### Examples
+
+Command with option | Environment variable
+------------------- | --------------------
+`toot --color` | `TOOT_COLOR=true`
+`toot --no-color` | `TOOT_COLOR=false`
+`toot post --editor vim` | `TOOT_POST_EDITOR=vim`
+`toot post --visibility unlisted` | `TOOT_POST_VISIBILITY=unlisted`
+`toot tui --media-viewer feh` | `TOOT_TUI_MEDIA_VIEWER=feh`
+
+Note that these can also be set via the [settings file](./settings.html).