README: Note on system SDL2 needs improvement

[eliheuer]

This is what I get running cargo build --features=sdl2-dynamic on MacOS 11:

MINI-M1 🌎 ~/MFEK/MFEKglif ➤ c874d5a|master✓
11323 ± : cargo build --features=sdl2-dynamic                                                [75d2h2m]
   Compiling MFEKglif v1.1.1-beta2 (/Users/eli/MFEK/MFEKglif)
error: failed to run custom build command for `MFEKglif v1.1.1-beta2 (/Users/eli/MFEK/MFEKglif)`

Caused by:
  process didn't exit successfully: `/Users/eli/MFEK/MFEKglif/target/debug/build/MFEKglif-15cad062b18f2a50/build-script-build` (exit status: 101)
  --- stdout
  cargo:rustc-env=MFEK_VERSION=v1.1.1b2-25-gc874d5a
  cargo:rustc-env=MFEK_COMPILED_AT=1661052412

  --- stderr
  thread 'main' panicked at '`sdl2-config` not in PATH: CannotFindBinaryPath', build.rs:50:47
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[eliheuer]

Running brew install sdl2 allows cargo build to finish.