Add ChangeLog entries for v2.1 release.

ChangeLog.txt

+* Thu Mar 20 2014 - Andreas Schuh <google-gflags@googlegroups.com>
+
+- gflags: version 2.1.0
+- Build system configuration using CMake instead of autotools
+- CPack packaging support for Debian/Ubuntu, Red Hat, and Mac OS X
+- Fixed issue 54: Fix "invalid suffix on literal" (C++11)
+- Fixed issue 57: Use _strdup instead of strdup on Windows
+- Fixed issue 62: Change all preprocessor include guards to start with GFLAGS_
+- Fixed issue 64: Add DEFINE_validator macro
+
 * Wed Jan 25 2012 - Google Inc. <google-gflags@googlegroups.com>
 
 - gflags: version 2.0