Update README.md

corrected var for disabling mirror
Bumps version to 0.2.15
Updates instructions for publishing
Removes Atmosphere publishing instruction section
Fixes for #114.

Adds support for starting the app form a subdirectory.
Creates mirror path when it doesn't exist when writing settings.json.
Calls mirror with absolute path to settings.json.
Adds info how to develop with a local version of velocity:core
Merge pull request #130 from meteor-velocity/fix-mirror-settings-path

Fixes for #114.
Numbered list for publishing steps
merged PR #130 into PR #129