docs: add documentation for extensions.objectFormat

Document the extensions.objectFormat config setting.  Warn users not to
modify it themselves.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -348,6 +348,8 @@ include::config/diff.txt[]
 
 include::config/difftool.txt[]
 
+include::config/extensions.txt[]
+
 include::config/fastimport.txt[]
 
 include::config/feature.txt[]

Documentation/config/extensions.txt

@@ -0,0 +1,8 @@
+extensions.objectFormat::
+	Specify the hash algorithm to use.  The acceptable values are `sha1` and
+	`sha256`.  If not specified, `sha1` is assumed.  It is an error to specify
+	this key unless `core.repositoryFormatVersion` is 1.
++
+Note that this setting should only be set by linkgit:git-init[1] or
+linkgit:git-clone[1].  Trying to change it after initialization will not
+work and will produce hard-to-diagnose issues.