PCF 434: Add documentation to ReadME

[Carla-Moz]

I've completed writing the documentation for PerfCompare and think it would help to add it to our README. I'd like to get input from the whole team if possible. Thanks.

PerfCompare Documentation

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.33%. Comparing base (b3ed6d2) to head (b3ba2e0). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #762 +/- ## ======================================= Coverage 91.33% 91.33% ======================================= Files 87 87 Lines 2309 2309 Branches 433 433 ======================================= Hits 2109 2109 Misses 177 177 Partials 23 23 ``` | [Flag](https://app.codecov.io/gh/mozilla/perfcompare/pull/762/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/mozilla/perfcompare/pull/762/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | `91.33% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla#carryforward-flags-in-the-pull-request-comment) to find out more.

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[esanuandra]

I'm not sure if displaying it in edit mode is right.

[Carla-Moz]

I'm not sure if displaying it in edit mode is right.

Our team has edit access; that's why you see it in edit mode. Everyone else sees it in viewer mode.