Restore CODING-GUIDELINES to doc folder

In r294740, the CODING-GUIDELINES was removed from the doc folder in favor
of the content on the Asterisk wiki.  Some folks still look in the doc folder
initially for coding guideline suggestions; as such, this patch adds a
CODING-GUIDELINES file back into the doc folder.  The content of the file
merely points to the correct page on the Asterisk wiki where the coding
guidelines currently live.

(closes issue ASTERISK-20279)
Reported by: Andrew Latham
Patches:
  CODING-GUIDELINES.diff uploaded by Andrew Latham (license 5985)
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Merged revisions 371961 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371962 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371963 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371964 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/CODING-GUIDELINES

+Coding guidelines are available on the Asterisk wiki at:
+https://wiki.asterisk.org/wiki/display/AST/Coding+Guidelines