Clarify Source events in Readme

[aquarion]

Description

Split the Javascript events into two lists, and explicitly stated what the Source events react to.

Motivation and Context

The Source events aren't documented with the rest of the events in the OBS source, and the Readme needs a bit of clarification that they're only firing when the browser source changes.

This was suprising to me, so I'm documenting it to avoid suprising others.

How Has This Been Tested?

Text change. Checked for spelling changes.

Types of changes

Documention change.

Checklist:

• [/] My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.