add note on index search to docs

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

docs/GOPHER.md

@@ -96,6 +96,23 @@ Titles are sent as `i<title text>\tTITLE\tnull.host\t0`.
 Web address links are sent as `h<text here>\tURL:<address>\thostname\tport`.
 An HTML redirect is sent in response to any requests beginning with `URL:`.
 
+## Index search type
+
+Index searches are split at the first tab in the request, and appended to the
+params string of the request. Gophi does not handle index searches directly,
+but the content of an index search query can be found in the `$QUERY_STRING`
+CGI environment variable, it is prepended to any other params found and should
+be set AFTER any query parameters otherwise the query params will be set as-is
+for the `$QUERY_STRING` variable.
+
+E.g.
+
+`/cgi-bin/search.cgi\tsearch%20query%20here` will set:
+`QUERY_STRING='search query here'`
+
+`/cgi-bin/search.cgi?key=value\tsearch query here` will set:
+`QUERY_STRING='key=value&search query here'`
+
 ## Policy files
 
 Upon request, `caps.txt` can be provided from the server root directory