Environment variables docs

docs/env.md

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+# Configuring
+
+txtdot can be configured either with environment variables
+or with the `.env` file in the working directory which has higher priority.
+For sample config, see [`.env.example`](https://github.com/TxtDot/txtdot/blob/main/.env.example).
+
+## HOST
+
+Default: `0.0.0.0`
+
+Host where HTTP server should listen for connections.
+Set it to `127.0.0.1` if your txtdot instance is behind reverse proxy,
+`0.0.0.0` otherwise.
+
+## PORT
+
+Default: `8080`
+
+Port where HTTP server should listen for connections.
+
+## REVERSE_PROXY
+
+Default: `false`
+
+Set it to `true` only if your txtdot instance runs behind reverse proxy.
+Needed for processing X-Forwarded headers.