Added instruction to keep the product description to 132 characters or less.

This is a restriction imposed by Chrome Web Store.

R=garykac@chromium.org

Review URL: https://codereview.chromium.org/23708059

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224533 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224533