Update distribution date and readme

It sync with the RC.

Marlin/Version.h

  * here we define this default string as the date where the latest release
  * version was tagged.
  */
-#define STRING_DISTRIBUTION_DATE "2016-04-16 12:00"
+#define STRING_DISTRIBUTION_DATE "2016-04-24 12:00"
 
 /**
  * @todo: Missing documentation block

README.md

 <img align="top" width=175 src="Documentation/Logo/Marlin%20Logo%20GitHub.png" />
  Additional documentation can be found in [The Marlin Wiki](https://github.com/MarlinFirmware/Marlin/wiki).
 
-## Release Candidate -- Marlin 1.1.0-RC5 - 01 April 2016
+## Release Candidate -- Marlin 1.1.0-RC6 - 24 April 2016
 
 __Not for production use – use with caution!__
 
 
 ## Recent Changes
 - RCBugFix
-  - Throw error if compiling with older versions (<1.60) of Arduino due to serios problems with outdated Arduino versions
+  - Throw error if compiling with older versions (<1.60) of Arduino due to serious problems with outdated Arduino versions
   - Please upgrade your IDE at least to Arduino 1.6.0. Thanks.
 
+- RC6 - 23 Apr 2016
+  - Completed support for CoreXY / CoreXZ in planner
+  - Changes to positioning behavior
+  - Various issues fixed. More details pending.
+
 - RC5 - 01 Apr 2016
   - Warn if compiling with older versions (<1.50) of Arduino
   - Fix various LCD menu issues
 
 While we can't prevent the use of this code in products (3D printers, CNC, etc.) that are closed source or crippled by a patent, we would prefer that you choose another firmware or, better yet, make your own.
 
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