Add notes on creating a self signed build (#13987)
Summary of the issue:
Without a signed NVDA build, certain features cannot be tested.
Only NV Access have the ability to sign a build with the NV Access certificate.
Other developers who wish to create a signed build to test NVDA must sign the build with their own certificate.
There is no documentation on creating a self-signed build.
Description of user facing changes
Added notes on creating and testing a self-signed build.
Description of development approach
Add notes, tested creating a self-signed build.
Based on these instructions: https://docs.microsoft.com/en-us/windows/msix/package/create-certificate-package-signing
