Add remark how to fix weird git subtree pull issues

INSTALL.md

@@ -42,6 +42,15 @@ To update a dependency project (e.g. mamba) subtree please do
 git subtree pull --prefix=deps/mamba --squash git@gitlab.com:cerl/mamba.git master
 ```
 
+If `git` refuses to do the subtree update insisting 'repo was never added', create a remote for the repo, e.g.,
+
+```
+git remote add -f ofiwg  git@github.com:ofiwg/libfabric.git 
+```
+
+and use that repo name for the `subtree pull` command.
+
+
 
 # Building