Update index.srt

Added details about "Improve this Doc" button and completed the offline options description.
Merge pull request #1756 from blackpuppy/zh

[zh_CN] add basic installation & cakephp-overview
[pt] [2.x] Update index.rst

Added details about "Improve this Doc" button.
Corrected the uppercase mistake in one of the links.
Completed the offline options description.
Merge pull request #1752 from andtxr/patch-1

[pt] [2.x] Update index.srt
Merge branch 'master' into zh