[Release] Instructions for updating to/staying on v10/v11

[snv]

use the config from here:

@snv

[core]
       repositoryformatversion = 0
       filemode = true
       bare = false
       logallrefupdates = true
[remote "upstream"]
       url = https://github.com/frappe/erpnext
       fetch = +refs/heads/*:refs/remotes/upstream/*
[branch "staging"]
       remote = upstream
       merge = refs/heads/staging