Add command to bump readme.txt to stable tag

[r-a-y]

When a new version of WordPress is released, WP wants plugins to bump their readme.txt so their Tested up to: header matches the latest WP release.

If we just want to bump this header for WP compatibility, I think we should add a simple command to wp-release.conf like BUMP_README (similar to DRYRUN).

If BUMP_README is enabled, this would copy the readme.txt from Git's master branch to the plugin SVN repo's current tagged directory.

aubreypwd is doing something like this here:
https://github.com/aubreypwd/deploy-git-wordpress-org/blob/master/deploy-git-wordpress-org.sh#L75-L97

I'll add a pull request a bit later once I've tested things.