Add a note about using a predates_sharding_branch branch

[orta]

This should fix https://github.com/CocoaPods/CocoaPods/issues/6167 - I've pushed a branch to the Specs repo so that a git pull won't fail for 0.39 users with the master repo, which should make the update function work.

For those without the Specs repo we might have to add some extra instructions?

@segiddins - this should work right?

[orta]

Good point, I previously was branching locally from a commit

[orta]

Is this good to go?

[segiddins]

Well it already pulls master, you need to use --no-repo-update if on 0.39

[orta]

Alright, I've updated that and a added a note about no repo update