README: rename to README.md, add markdown for github

Change-Id: I2e644aa2d693692f33d017c00367a734039532f1

README.md

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+This is a port of many Plan 9 libraries and programs to Unix.
+
+* Installation
+
+To install, run ./INSTALL.  It builds mk and then uses mk to
+run the rest of the installation.  
+
+For more details, see install(1), at install.txt in this directory
+and at http://swtch.com/plan9port/man/man1/install.html.
+
+* Documentation
+
+See http://swtch.com/plan9port/man/ for more documentation.
+(Documentation is also in this tree, but you need to run
+a successful install first.  After that, "9 man 1 intro".)
+
+Intro(1) contains a list of man pages that describe new features
+or differences from Plan 9.
+
+* Helping out
+
+If you'd like to help out, great!  The TODO file contains a small list.
+
+If you port this code to other architectures, please share your changes
+so others can benefit.
+
+Please use diff -u or CVS (see below) to prepare patches.
+
+* CVS
+
+You can use CVS to keep your local copy up-to-date as we make 
+changes and fix bugs.  See the cvs(1) man page here ("9 man cvs")
+for details on using cvs.
+
+* Contact
+
+Russ Cox <rsc@swtch.com>