document the config file

README.md

@@ -12,6 +12,16 @@ other uses I didn't think of.
 The uploaded files have a lifetime of 10 minutes, which can be shortened or 
 extended up to 1 hour (or more, as configured).
 
+## Config file
+
+The server application reads a config file, called `postit.json` by default.
+The file path can be set by a CLI argument (see `--help`).
+
+Call the binary with `--default-config` to dump the default JSON. See `src/config.rs`
+for documentation of the format.
+
+The JSON file uses the JSON5 format - comments are allowed.
+
 ## Uploading a file
 
 To upload a file, send a POST request to the running PostIt server.