add Manifest.toml to .gitignore

[Fe-r-oz]

This PR aims to add Manifest.toml to .gitignore. This will allow users to add the packages without having to worry about changes in Manifest.toml that shows up on PR.

[davidamarquis]

I appreciate you want to improve the workflow and I like the sound of this change. Compared with a normal pull request, syncing git workflow changes across multiple people involves a bit of extra work.

If we're going to do this change then there's a few more things you need to do, I think:

1. find the git command for stopping tracking a file that's already tracked and add it to the PR description above
2. communicate this PR on the Slack to make sure everyone who makes PRs knows that when they update their local working directory from the remote dev branch that they need to run the command in step 1)
3. remove the Manifest.toml file from CT in this PR

If a person who makes PRs doesn't do 2) then Manifest.toml is just going to be slipped back in on their next PR.

There may be other steps I haven't thought of. Its worth taking the time to think about because its an opportunity to learn more about git.

[Fe-r-oz]

Thank you for your positive reaction and your feedback! That is much appreciated!