Add Mercurial as a category and add a Mercurial Course. (#2575)

* Add Mercurial as a category and add a Mercurial Course.

* Removed trailing slash

free-courses-en.md

@@ -25,6 +25,7 @@
 * [Lua](#lua)
 * [Machine Learning](#machine-learning)
 * [Matlab](#matlab)
+* [Mercurial](#mercurial)
 * [Misc](#misc)
 * [.NET](#net)
 * [OCaml](#ocaml)
@@ -218,6 +219,11 @@
 * [MIT 18.S997 Introduction to MATLAB Programming](http://ocw.mit.edu/courses/mathematics/18-s997-introduction-to-matlab-programming-fall-2011/)
 
 
+### Mercurial
+
+* [Hg Init: a Mercurial Tutorial](http://hginit.com) - Joel Spolsky
+
+
 ### Misc
 
 * [Advanced Data Structures](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-851-advanced-data-structures-spring-2012/)