glossary: add "remote", "submodule", "superproject"

Noticed-by: Philip Oakley <philipoakley@iee.org>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -469,6 +469,11 @@ The most notable example is `HEAD`.
 	<<def_push,push>> to describe the mapping between remote
 	<<def_ref,ref>> and local ref.
 
+[[def_remote]]remote repository::
+	A <<def_repository,repository>> which is used to track the same
+	project but resides somewhere else. To communicate with remotes,
+	see <<def_fetch,fetch>> or <<def_push,push>>.
+
 [[def_remote_tracking_branch]]remote-tracking branch::
 	A <<def_ref,ref>> that is used to follow changes from another
 	<<def_repository,repository>>. It typically looks like
@@ -515,6 +520,17 @@ The most notable example is `HEAD`.
 	is created by giving the `--depth` option to linkgit:git-clone[1], and
 	its history can be later deepened with linkgit:git-fetch[1].
 
+[[def_submodule]]submodule::
+	A <<def_repository,repository>> that holds the history of a
+	separate project inside another repository (the latter of
+	which is called <<def_superproject, superproject>>).
+
+[[def_superproject]]superproject::
+	A <<def_repository,repository>> that references repositories
+	of other projects in its working tree as <<def_submodule,submodules>>.
+	The superproject knows about the names of (but does not hold
+	copies of) commit objects of the contained submodules.
+
 [[def_symref]]symref::
 	Symbolic reference: instead of containing the <<def_SHA1,SHA-1>>
 	id itself, it is of the format 'ref: refs/some/thing' and when