Explain Trade-offs of Different Modes in the README

[gkapfham]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have read the README.md and believe that my question is not covered.

Issue

The current version of the documentation mentions that CommitCanvas operates in
both a project-specific and a project-agnostic mode. However, the distinctions
between these two modes is not clear after reading the documentation. As such,
it would be beneficial for the README file to include more details about when a
developer would pick one mode over the other. If possible, the README should
also include some information about the performance of the different modes.
Finally, the documentation should reflect the correct model that is used in the
project-agnostic mode for CommitCanvas.