[docs] Add gmake references to compilations instructions to support FreeBSD

This is just a short note for FreeBSD users they want to compile this project. I compiled the project successfully [on FreeBSD v13.3] according to the instruction [Compile from source code](https://github.com/lightninglabs/lightning-terminal/blob/master/docs/compile.md#compile-from-source-code). The only difference is to use `gmake install` instead of `make install`.