add installation instructions to pkg packages
add installation instructions to pkg packages

BUG=

Review URL: https://codereview.chromium.org//14188048

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@21770 260f80e4-7a28-3924-810f-c04153c831b5

thosakwe/oauth2,dart-lang/oauth2,ErikGrimes/oauth2