More docs in travis config

.travis.yml

@@ -14,6 +14,10 @@ install:
 - ${CXX} -v
 addons:
   apt:
+    # List of whitelisted in travis packages for ubuntu-precise can be found here:
+    #   https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-precise
+    # List of whitelisted in travis apt-sources:
+    #   https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json
     sources:
     - ubuntu-toolchain-r-test
     - llvm-toolchain-precise-3.7