Add guide for third-party build system implementers

[DanAlbert]

ndk-build changes often enough that it's a hassle for maintainers of third-party build systems to keep up unless I'm actively notifying them of new features.

I should add a doc describing the parts of ndk-build that are relevant to implementers of third-party build systems.

Information about unified headers would be a good thing to start with.

[DanAlbert]

I'm not really sure how much there will be to write once we reach r19, since at that point the doc will just be "call clang as if it was any other system". This may end up just getting closed.

[DanAlbert]

A draft is now up for review: https://android-review.googlesource.com/c/platform/ndk/+/750953

Note that this is written for the not-yet-released (not even merged yet) r19 toolchain changes. Some of the pieces are still valid for r18 and before, but many of the details (and all of the paths) are incorrect.

Still, if you're wondering what you'll need to do to adapt to the new r19 toolchain (mostly delete Android-specific code), it's worth a read.

[DanAlbert]

https://android.googlesource.com/platform/ndk/+/master/docs/BuildSystemMaintainers.md

NB: As I said above, this doc (mostly) isn't helpful for any currently released NDK. This starts being useful when r19 ships to beta.