buildbucket: added readme
buildbucket: added readme

R=sergeyberezin@chromium.org, vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/835943002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293521 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build