Documentation: note styling for bit fields

Our codebase uses a lot of bit field variables, generally to mark
boolean type variables. While there is a formatting rule in the
'.clang-format', there is no guideline specified in the
'CodingGuidelines'.

Since the '.clang-format' is not yet enforced, let's also add a
guideline with the same rule as mentioned in the '.clang-format', which
is to not use any spaces around the colon, like so:

    unsigned my_field:1;
    unsigned other_field:1;
    unsigned field_with_longer_name:1;

This would allow us not to modify the clang-format file, and more
importantly, discourage people from doing ugly alignment with spaces,
i.e.

    unsigned my_field               : 1;
    unsigned            other_field : 1;
    unsigned field_with_longer_name : 1;

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -650,6 +650,12 @@ For C programs:
    cases. However, it is recommended to find a more descriptive name wherever
    possible to improve the readability and maintainability of the code.
 
+ - Bit fields should be defined without a space around the colon. E.g.
+
+   unsigned my_field:1;
+   unsigned other_field:1;
+   unsigned field_with_longer_name:1;
+
 For Perl programs:
 
  - Most of the C guidelines above apply.