Revise the warning in the README

[rstacruz]

From README.rdoc:

WARNING
Note that this code will blow away the initial/final comment block in your models if it looks like it was previously added by this gem, so you don’t want to add additional text to an automatically created comment block.

BACK UP YOUR MODELS BEFORE USING THIS TOOL!

Lol, "back up". Who builds projects without version control, nowadays? Something like this might be a better fit:

Be sure to check the changes that this tool makes! If you are using Git, you may simply check your project's status after running annotate:

$ git status

If you are not using a VCS (like Git, Subversion or similar), please tread extra carefully, and consider using one.