chore: add CODEOWNERS.md and README.md to apm-python team (#9889)

These two .md files should be approvable by anyone in apm-python, not
just core.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

.github/CODEOWNERS

@@ -19,6 +19,8 @@ releasenotes/                       @DataDog/apm-python
 tests/snapshots/                    @DataDog/apm-python
 riotfile.py                         @DataDog/apm-python
 .riot/requirements/                 @DataDog/apm-python
+CHANGELOG.md                        @DataDog/apm-python
+README.md                           @DataDog/apm-python
 
 # Core / Language Platform
 tests/internal                      @DataDog/apm-core-python