Add proposals doc

PROPOSALS.md

@@ -0,0 +1,44 @@
+This page is also available at [gemini://twins.rocketnine.space/proposals.gmi](gemini://twins.rocketnine.space/proposals.gmi)
+
+The following proposals add fields to the response header of successful
+requests.
+
+Standard successful response header (for comparison):
+
+```
+20 text/gemini; charset=utf-8
+```
+
+These additions are backwards-compatible as specified in [RFC 2045](https://tools.ietf.org/html/rfc2045):
+
+> MIME implementations must ignore any parameters whose names they do not recognize.
+
+The terms MAY, SHOULD and SHOULD NOT are defined in [RFC 2119](https://tools.ietf.org/html/rfc2119).
+
+# Size
+
+Gemini servers SHOULD include the size (in bytes) of the response body when the
+request is successful. Clients SHOULD utilize this information when downloading
+files to indicate progress.
+
+```
+20 text/gemini; charset=utf-8; size=1108
+```
+
+# Cache
+
+Gemini servers MAY include a duration (in seconds) which a client SHOULD cache
+a resource when the request is successful. When a cache duration of 0 or
+less is provided, clients SHOULD NOT cache the resource.
+
+Cache for one week:
+
+```
+20 text/gemini; charset=utf-8; cache=604800
+```
+
+Do not cache:
+
+```
+20 text/gemini; charset=utf-8; cache=0
+```

README.md

@@ -19,6 +19,10 @@ This page is also available at [gemini://twins.rocketnine.space](gemini://twins.
 - Serve system command output
 - Reload configuration on `SIGHUP`
 
+## Proposals
+
+twins includes features that are not yet part of the Gemini specification. See [PROPOSALS.md](https://gitlab.com/tslocum/twins/blob/master/PROPOSALS.md)
+
 ## Download
 
 ```bash