doc/contributing: New file

Partly inspired by GNU coreutils HACKING[*].

doc/coding section "git" is now redundant, except for the note on
avoiding whitespace changes.  Move that to section "Code formatting",
and delete section "git".

[*] http://git.savannah.gnu.org/cgit/coreutils.git/plain/HACKING?id=77da73c

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

doc/coding

@@ -44,6 +44,8 @@ These guidelines don't attempt to be exhaustive.  More complete
 guidelines that are mostly compatible with Empire can be found at
 http://www.jetcafe.org/~jim/c-style.html
 
+See also doc/contributing.
+
 
 Source tree organization
 ------------------------
@@ -78,6 +80,12 @@ you set the style for the current buffer.  Set variable
 `c-default-style' to "stroustrup" to switch to this style for all
 buffers.
 
+Avoid gratuitous space change
+
+Don't change whitespace gratuitiously, say just because your editor
+screws up tabs.  Such changes make it much harder to figure out who
+changed what and when.
+
 Tab character use
 
 Whether you use tab characters or not doesn't really matter that much,
@@ -462,24 +470,6 @@ Others return the same error value for failed read and for successful
 read of input that is invalid.  Then you need to check
 player->aborted; if it is set after a read, the read failed.
 
-
-git
----
-
-Commit related changes together, unrelated changes separately.
-
-Write meaningfull commit messages.  Start with a single short line
-(ideally less than 50 characters) summarizing the change, followed by
-a blank line and then a more thorough description.
-
-The purpose of the change log is not to explain how the code works;
-that should be done in the source code itself.  It's to explain *why*
-you made the change, and what is affected by it.
-
-Don't change whitespace gratuitiously, say just because your editor
-screws up tabs.  Such changes make it much harder to figure out who
-changed what and when.
-
 
 Historical guidelines, superseded by the above