Add uploading section to README.

[fuzzybinary]

What does this PR do?

Add an Uploading section for the README which includes instructions for uploading mapping files and NDK symbol files

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [ ] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

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

Project coverage is 77.68%. Comparing base (32fa206) to head (e3e6dbf). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #281 +/- ## ============================================= - Coverage 78.01% 77.68% -0.33% + Complexity 251 248 -3 ============================================= Files 31 31 Lines 914 914 Branches 114 114 ============================================= - Hits 713 710 -3 - Misses 148 150 +2 - Partials 53 54 +1 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/DataDog/dd-sdk-android-gradle-plugin/pull/281/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog)