Flesh out README.

README.md

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 # AV-98
 
-Experimental VF-1-derived client for the  "Gemini" protocol
+AV-98 is an experimental client for the
+[Gemini protocol](https://gemini.circumlunar.space).  It is derived from the
+[gopher client VF-1](https://github.com/solderpunk/VF-1) by the same author.
+AV-98 is "experimental" in the sense that it may occasionally extend or deviate
+from the official Gemini specification for the purposes of, well,
+experimentation.  Despite this, it is expected to be stable enough for regular
+daily use at the same time.
+
+## Dependencies
+
+AV-98 has no "strict dependencies", i.e. it will run and work without anything
+else beyond the Python standard library.  However, it will "opportunistically
+import" a few other libraries if they are available to offer an improved
+experience.
+
+* The [ansiwrap library](https://pypi.org/project/ansiwrap/) may result in
+  neater display of text which makes use of ANSI escape codes to control colour.
+* The [cryptography library](https://pypi.org/project/cryptography/) will
+  provide a better and slightly more secure experience when using the default
+  TOFU certificate validation mode and is highly recommended.
+
+## Features
+
+* TOFU or CA server certificate validation
+* Extensive client certificate support if an `openssl` binary is available
+* Ability to specify external handler programs for different MIME types
+* Gopher proxy support (e.g. for use with
+  [Agena](https://tildegit.org/solderpunk/agena))
+* Advanced navigation tools like `tour` and `mark` (as per VF-1)
+* Bookmarks
+* IPv6 support
+* Supports any character encoding recognised by Python
+
+## Lightning introduction
+
+You use the `go` command to visit a URL, e.g. `go gemini.circumlunar.space`.
+
+Links in Gemini documents are assigned numerical indices.  Just type an index to
+follow that link.
+
+If a Gemini document is too long to fit on your screen, use the `less` command
+to pipe it to the `less` pager.
+
+Use the `help` command to learn about additional commands.