Document how to get working on Apple M1 + Homebrew v3

[niftylettuce]

On homebrew v3 the default path is /opt/homebrew, which is not included in clang search paths by default. Therefore this project will fail to build. I added the following to my zshrc/bashrc on my Apple M1 https://github.com/Homebrew/brew/issues/10152#issuecomment-774706744 and it did the trick.

[niftylettuce]

Ref: https://github.com/kaworu/node-quirc/issues/16

[kaworu]

Hello @niftylettuce, thanks for opening this issue!

On homebrew v3 the default path is /opt/homebrew, which is not included in clang search paths by default. Therefore this project will fail to build.

If I understand correctly, this problem will arise with any project. In other words, this is an environment issue unrelated to quirc.

In my humble opinion, based on the fact that we're only providing a Makefile rather than using a build system (e.g. autotools or CMake), users of this library are expected to understand the basic of their compiler toolchain.