Add build instructions for libvpx

[curtisy1]

For a first time installation on develop, recurse submodules is necessary, on top of building libvpx. Otherwise, building the app will fail.

Add a paragraph to address this

[gdlbo]

Maybe it would be better to do it as a Gradle script?

[curtisy1]

Good idea. I'll close this and spin up a new PR adding this into a Gradle script.
Do you prefer keeping the build.sh file or should I move the logic to Gradle entirely? Personally, I'd lean towards keeping it in bash to avoid cluttering the gradle file

[aliveoutside]

Maybe it would be better to do it as a Gradle script?

Strong disagree. Just keep git clone --recurse-submodules in README. Don't break separation of concerns