add a contributing document

text is pulled out of the wiki.  This will enable github to show a link when
a contributor creates an issue or opens a Pull Request
list-memberships and perms changes:

  - list-memberships now requires a '-r' or '-u'; i.e., you have to
    explicitly state whether you are passing a reponame or a username

    see the new '-h' message for details.

  - Easy.pm now has a new 'in_role()' test that is, at present, only
    used by 'owns()', which uses that instead of checking that he is the
    creator.

    The role name used (I recommend "OWNER") must be set in the rc file
    like so

        OWNER_ROLENAME => 'OWNER',

    and if it is not set, defaults to 'CREATOR', which makes it behave
    as things currently do.

  - perms now uses this new 'owns()' function to authorise the user,
    instead of checking that she is the *creator*
new command 'owns' (thanks to Kevin Pulo)

(See 'owns2' in
https://groups.google.com/d/msg/gitolite/InJDugkXY0s/Lxa_gsd_WbUJ)
DEFAULT_ROLE_PERMS replaced by per repo 'default.roles' option
overwrite projects.list atomically

avoids any potential race conditions between triggers being run
concurrently, and probably avoids gitweb picking up a half-done file too
v3.5
(bugfix) (v3.5.1) fix permissions of projects.list file

update-gitweb-access-list had a change recently (289b19d) that had a
side-effect of making projects.list unreadable by other userids (like
'apache' or whatever).

This makes 3.5 unusable for anyone using gitweb.  (Hence the new tag,
too, though 3.5 is only the previous commit and this is a very small
change).