update readme with config instructions and stuff

README.md

@@ -2,10 +2,20 @@
 
 ## run it
 
-    npm i
+no external dependencies! no need to `npm i`! woo!
+
     cp webring.txt.sample webring.txt && $EDITOR webring.txt
     npm run start
 
+## configuration
+
+the server listens on port `$PORT` (default 80) and loads the list of
+participating sites from the file `$SITES_FILE` (default `webring.txt`).
+
+the sites file is a newline-separated list of URLs; leading and trailing
+whitespace, empty lines, and lines starting with `#` are ignored. you can test
+out the format using [this regexr link](https://regexr.com/?expression=/^\s*([^%23\s].*)\s*$/gm&text=%23%20sample%20SITES_FILE%0A%0A%23%20my%20friend%20alice%20who%20is%20cool%0Ahttps%3A%2F%2Falices.awesome.website%0A%0A%23%20my%20other%20friend%20bob%20who%20is%20neat%20as%20well%0Ahttp%3A%2F%2Fsome.shared.domain%2F~bob%0A&tool=list&input=your.webring.server/next?from=$1\n).
+
 ## link to webring sites
 
 participating sites should link to the endpoints `/prev?from=<site>` and