Add Limitation section for pinning_version
added gpg key for apt repository

Isn't it better to just add the gpg key to the apt repository instead of forcing yes on the package install without gpg key?
Merge pull request #19 from Raboo/patch-1

added gpg key for apt repository
Add support for platform "amazon" in accordance to my agreement with treasure-data/chef-td-agent current
maintainer, repeatedly@gmail.com.  As per issue, Uneccesarly strick platform check #18,
  "We don't have a test environment for Amazon Linux and we don't guarantee td-agent works on Amazon Linux.
  If it is OK, please send a PR :)"
Done.

thanks!
Merge pull request #21 from benself/master

Add support for platform "amazon" (tested)
Add notice for Amazon Linux environment
Update README.rdoc

change wording a bit.
Merge pull request #22 from ijin/patch-1

Update README.rdoc