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 ✅

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

Additional details and impacted files

@@              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

[michaelcretzman]

I recommend adding the suggested change by Oxnm.