Given the many upvotes on this post on StackOverflow https://stackoverflow.com/questions/47455300/ssh-config-bad-configuration-option-usekeychain-on-mac-os-sierra-10-12-6 it seems like many users have an issue with Mac not dealing properly with the UseKeychain config in the ~/.ssh/config. This commit adds additional information on how to fix the problem and allows GitHub users to find a solution in the GitHub documentation when searching for the error message.

Why:

Given the many upvotes on this post on StackOverflow https://stackoverflow.com/questions/47455300/ssh-config-bad-configuration-option-usekeychain-on-mac-os-sierra-10-12-6 it seems like many users have an issue with Mac not dealing properly with the UseKeychain config in the ~/.ssh/config. This commit adds additional information on how to fix the problem and allows GitHub users to find a solution in the GitHub documentation when searching for the error message.

What's being changed:

Add additional information for Mac users on how to fix the ~/.ssh/config configuration on their Macs. Diff should be self-explanatory.

Check off the following:

• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (eg. a "before and after video")