Add documentation for ctrl+i keybinding

[d-tork]

I couldn't figure out how to get back in a cell from Jupyter command mode, until I searched the repo and found the ctrl+i change made in the history file. This just puts the keybinding on the front page so new users can find it.

Please finish the following when submitting a pull request:

• [x] Add a line to History.md briefly documenting the change.

If this is a release, additionally do the following:

• [ ] Bump the package version in package.json
• [ ] Update the dependencies in package.json
• [ ] Run jlpm install to update yarn.lock

Thanks!