Add gistic-cohort-vs-histology-comparison README

[cbethell]

Purpose/implementation Section

The purpose of this PR is to add a README.md file to the gistic-cohort-vs-histology-comparison module to outline what was done in PR #549.
This PR also adds the description of this analysis to the analyses/README.md file

What scientific question is your analysis addressing?

This is a documentation PR.

What was your approach?

I summarized what was done in PR #549.

What GitHub issue does your pull request address?

This PR addresses issue #547.

Directions for reviewers. Tell potential reviewers what kind of feedback you are soliciting.

Which areas should receive a particularly close look?

Specifically, the analyses/README.md changes, to ensure that I followed the correct format there.

Is the analysis in a mature enough form that the resulting figure(s) and/or table(s) are ready for review?

Yes, this is ready for review.

Reproducibility Checklist

• The dependencies required to run the code in this pull request have been added to the project Dockerfile.
• This analysis has been added to continuous integration.

Documentation Checklist

• This analysis module has a README and it is up to date.
• This analysis is recorded in the table in analyses/README.md and the entry is up to date.
• The analytical code is documented and contains comments.

[jaclyn-taroni]

👍 LGTM