[U-Boot-Users] Revised custodian git writeup

http://www.denx.de/wiki/view/UBoot/CustodianGitTrees#Tips_for_maintaining_custodian_t

I've rewritten the "Tips for maintaining custodian trees" section to reflect Wolfgang's request that the "master" branch be used for patches for him to pull.

Conceptually, this is very different from my previous recommendation / methodology writeup. In practice, however, it is a trivial change. The fundamental difference is to create a separate branch ("uboot") to track the master repo and rebase the "master" branch against that, instead of vice versa.

Best regards, gvb