Document master.cfg.txt
Document master.cfg.txt

rust-lang/rust-buildbot,rust-lang-deprecated/rust-buildbot,rust-lang/rust-buildbot,rust-lang/rust-buildbot,edunham/rust-buildbot,rust-lang-deprecated/rust-buildbot,edunham/rust-buildbot,edunham/rust-buildbot,rust-lang-deprecated/rust-buildbot