Update README, convert to markdown

With Gordon ending development of WiringPi, this repository needs to reflect the situation accurately and differentiate between Gordon's final code release and updates made since then.

README.TXT

@@ -1,36 +0,0 @@
-Note
-====
-
-This is an unofficial mirror of WiringPi to support ports (Python/Ruby/etc).
-
-Please do not email Gordon if you have issues, he will not be able to help.
-
-Pull-requests are not currently accepted, since this is a mirror.
-
-For support, comments, questions, etc please join the WiringPi Discord channel: https://discord.gg/SM4WUVG
-
-wiringPi README
-===============
-
-Please note that the official way to get wiringPi is via git from
-git.drogon.net and not GitHub.
-
-ie.
-
-  git clone git://git.drogon.net/wiringPi
-
-The version of wiringPi held on GitHub by "Gadgetoid" is used to build the
-wiringPython, Ruby, Perl, etc. wrappers for these other languages. This
-version may lag the official Drogon release.  Pull requests may not be
-accepted to Github....
-
-Please see
-
-  http://wiringpi.com/
-
-for the official documentation, etc. and the best way to submit bug reports, etc.
-is by sending an email to projects@drogon.net
-
-Thanks!
-
-  -Gordon

README.md

@@ -0,0 +1,19 @@
+Unofficial WiringPi Mirror
+==========================
+
+This is an unofficial mirror of WiringPi to support ports (Python/Ruby/etc).  With the
+[end of official development](http://wiringpi.com/wiringpi-deprecated/), this repository
+has become a mirror of the last "official" source release as well as a source for small
+updates to support newer hardware (primarily for use by the ports).
+
+  * The final "official" source release can be found at the
+    [`final_source_2.50`](https://github.com/WiringPi/WiringPi/tree/final_official_2.50) tag.
+  * The default `master` branch contains code that has been written since that final source
+    release to provide support for newer hardware.
+
+Please do not email Gordon if you have issues, he will not be able to help.
+
+Pull-requests may be accepted to add or fix support for newer hardware, but new features or
+other changes will not be accepted.
+
+For support, comments, questions, etc please join the WiringPi Discord channel: https://discord.gg/SM4WUVG