git: Add README.txt info file about in-tree dependencies

Signed-off-by: Paul Cercueil <[email protected]>

Author: pcercuei

deps/README.txt

@@ -0,0 +1,14 @@
+Commits of the Libiio repository must *not* touch any file or directory
+present in the deps/ folder.
+
+The only commits allowed, are the one that are automatically created
+when adding a new dependency with:
+git subrepo clone https://... deps/new-dependency
+
+And the ones created when updating the external repository to the latest
+available with:
+git subrepo pull deps/my-dependency
+
+
+git-subrepo is an external tool that can be obtained here:
+https://github.com/ingydotnet/git-subrepo