docs for envs

docs/markdown/backend/env.md

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+# Environment
+
+> ⚠ Available since version `v1.3.0`
+
+Sometimes it's favorable not having the encryption keys in the config files.
+For that `autorestic` allows passing the backend keys as `ENV` variables, or through an env file.
+
+The syntax for the `ENV` variables is as follows: `AUTORESTIC_[BACKEND NAME]_KEY`.
+
+```yaml | autorestic.yaml
+backend:
+  foo:
+    type: ...
+    path: ...
+    key: secret123 # => AUTORESTIC_FOO_KEY=secret123
+```
+
+## Example
+
+This means we could remove `key: secret123` from `.autorestic.yaml` and execute as follows:
+
+```bash
+AUTORESTIC_FOO_KEY=secret123 autorestic backup ...
+```
+
+## Env file
+
+Alternatively `autorestic` can load an env file, located next to `autorestic.yml` called `.autorestic.env`.
+
+```| .autorestic.env
+AUTORESTIC_FOO_KEY=secret123
+```
+
+after that you can simply use `autorestic` as your are used to.
+
+> :ToCPrevNext