CodingGuidelines: spell out how we use grep in our scripts

Our scripts try to stick to fairly limited subset of POSIX BRE for
portability.  It is unclear from manual page from GNU grep which is GNU
extension and which is portable, so let's spell it out to help new people
to keep their contributions from hurting porters.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -53,6 +53,18 @@ For shell scripts specifically (not exhaustive):
  - We do not write the noiseword "function" in front of shell
    functions.
 
+ - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
+   [::], [==], nor [..]) for portability.
+
+   - We do not use \{m,n\};
+
+   - We do not use -E;
+
+   - We do not use ? nor + (which are \{0,1\} and \{1,\}
+     respectively in BRE) but that goes without saying as these
+     are ERE elements not BRE (note that \? and \+ are not even part
+     of BRE -- making them accessible from BRE is a GNU extension).
+
 For C programs:
 
  - We use tabs to indent, and interpret tabs as taking up to