Improve the lockfile version warning message

Correct a typo and improve the warning message that's shown when the
lock file was created with a newer version of CocoaPods.
Updated guidelines on submitting feature requests.
Merge pull request #1758 from wilkinsona/improve-version-warning

Improve the lockfile version warning message
Merge pull request #1759 from paulyoung/update-contributing-guidelines

Updated guidelines on submitting feature requests