Write README.

README.md

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 # gemini-demo-1
 
 Minimal but usable interactive Gemini client in < 100 LOC of Python 3
+
+## Rationale
+
+One of the original design criteria for the Gemini protocol was that
+"a basic but usable (not ultra-spartan) client should fit comfortably
+within 50 or so lines of code in a modern high-level language.
+Certainly not more than 100".  This client was written to gauge how
+close to (or far from!) that goal the initial rough specification is.
+
+## Capabilities
+
+This crude but functional client:
+
+* Has a minimal interactive interface for "Gemini maps"
+* Will print plain text in any encoding if it is properly declared in
+  the server's response header
+* Will handle binary files using programs specified in `/etc/mailcap`
+  (so you can, e.g. view images)
+* Will follow redirects
+* Will report errors
+* Does NOT DO ANY validation of TLS certificates
+
+It's a *snug* fit in 100 lines, but it's possible.  A 50 LOC client
+would need to be much simpler.
+
+## Usage
+
+Run the script and you'll get a prompt.  Type a Gemini URL (the scheme
+is implied, so simply entering e.g. `gemini.conman.org` will work) to
+visit a Gemini location.
+
+If a Gemini menu is visited, you'll see numeric indices for links, ala
+VF-1 or AV-98.  Type a number to visit that link.
+
+There is very crude history: you can type `b` to go "back".
+
+Type `q` to quit.