docs: document how to build in release mode

[vicb]

Useful especially on MacOS where the local dev is broken (sefgault 11) ATM.

The README is awlays where I look to get the build command so I think it would be helpful there.

Thanks @danlapid for the tip.

[fhanau]

Note that the command we use to compile the official release binaries is slightly different to account for platform-specific flags: release_macos adds -O3 and several linker options that reduce binary size, for example. I worry that making this change as-is will create confusion, as we tell people to build in a configuration that doesn't include -O3 for release binaries (Windows needs /clang:-O3 instead, that's why we don't enable -O3 in the base release configuration).