documentation: add documentation for 'git version'

While 'git version' is probably the least complex git command,
it is a non-experimental user-facing builtin command. As such
it should have a help page.

Both `git help` and `git version` can be called as options
(`--help`/`--version`) that internally get converted to the
corresponding command. Add a small paragraph to
Documentation/git.txt describing how these two options
interact with each other and link to this help page for the
sub-options that `--version` can take. Well, currently there
is only one sub-option, but that could potentially increase
in future versions of Git.

Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-version.txt

@@ -0,0 +1,28 @@
+git-version(1)
+==============
+
+NAME
+----
+git-version - Display version information about Git
+
+SYNOPSIS
+--------
+[verse]
+'git version' [--build-options]
+
+DESCRIPTION
+-----------
+With no options given, the version of 'git' is printed on the standard output.
+
+Note that `git --version` is identical to `git version` because the
+former is internally converted into the latter.
+
+OPTIONS
+-------
+--build-options::
+	Include additional information about how git was built for diagnostic
+	purposes.
+
+GIT
+---
+Part of the linkgit:git[1] suite

Documentation/git.txt

@@ -41,6 +41,10 @@ OPTIONS
 -------
 --version::
 	Prints the Git suite version that the 'git' program came from.
++
+This option is internaly converted to `git version ...` and accepts
+the same options as the linkgit:git-version[1] command. If `--help` is
+also given, it takes precedence over `--version`.
 
 --help::
 	Prints the synopsis and a list of the most commonly used