contrib: contacts: add documentation

Assuming that git-contacts may some day be promoted to a core git
command, the documentation is written and formatted as if it already
belongs in Documentation/ even though it presently resides in
contrib/contacts.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/contacts/git-contacts.txt

@@ -0,0 +1,94 @@
+git-contacts(1)
+===============
+
+NAME
+----
+git-contacts - List people who might be interested in a set of changes
+
+
+SYNOPSIS
+--------
+[verse]
+'git contacts' (<patch>|<range>|<rev>)...
+
+
+DESCRIPTION
+-----------
+
+Given a set of changes, specified as patch files or revisions, determine people
+who might be interested in those changes.  This is done by consulting the
+history of each patch or revision hunk to find people mentioned by commits
+which touched the lines of files under consideration.
+
+Input consists of one or more patch files or revision arguments.  A revision
+argument can be a range or a single `<rev>` which is interpreted as
+`<rev>..HEAD`, thus the same revision arguments are accepted as for
+linkgit:git-format-patch[1]. Patch files and revision arguments can be combined
+in the same invocation.
+
+This command can be useful for determining the list of people with whom to
+discuss proposed changes, or for finding the list of recipients to Cc: when
+submitting a patch series via `git send-email`. For the latter case, `git
+contacts` can be used as the argument to `git send-email`'s `--cc-cmd` option.
+
+
+DISCUSSION
+----------
+
+`git blame` is invoked for each hunk in a patch file or revision.  For each
+commit mentioned by `git blame`, the commit message is consulted for people who
+authored, reviewed, signed, acknowledged, or were Cc:'d.  Once the list of
+participants is known, each person's relevance is computed by considering how
+many commits mentioned that person compared with the total number of commits
+under consideration.  The final output consists only of participants who exceed
+a minimum threshold of participation.
+
+
+OUTPUT
+------
+
+For each person of interest, a single line is output, terminated by a newline.
+If the person's name is known, ``Name $$<user@host>$$'' is printed; otherwise
+only ``$$<user@host>$$'' is printed.
+
+
+EXAMPLES
+--------
+
+* Consult patch files:
++
+------------
+$ git contacts feature/*.patch
+------------
+
+* Revision range:
++
+------------
+$ git contacts R1..R2
+------------
+
+* From a single revision to `HEAD`:
++
+------------
+$ git contacts origin
+------------
+
+* Helper for `git send-email`:
++
+------------
+$ git send-email --cc-cmd='git contacts' feature/*.patch
+------------
+
+
+LIMITATIONS
+-----------
+
+Several conditions controlling a person's significance are currently
+hard-coded, such as minimum participation level (10%), blame date-limiting (5
+years), and `-C` level for detecting moved and copied lines (a single `-C`). In
+the future, these conditions may become configurable.
+
+
+GIT
+---
+Part of the linkgit:git[1] suite