Explain build warnings

[fulldecent]

People using secp256k1 in their projects are used to seeing many build errors.

It will be helpful if a note here in the README explains why these build errors can be safely ignored.

[fulldecent]

-OR- what resources you need to help handle those warnings.

[fanatid]

PR's are welcome. I planning update project to n-api and add prebuilds, but no ETA right now. Most probably in January 2020.

[fanatid]

n-api version in fanatid/fcrypto

[fanatid]

New version 4.0.0 with n-api prebuilds released. Targets split with better flags, so even compilation will be required, there should not be warnings.
#160