move usage info to README

README.md

+ESP32 compiler for NixOS, build according to [this](https://docs.espressif.com/projects/esp-idf/en/latest/esp32/get-started/linux-setup-scratch.html) documentation using crosstool-NG.
+
+* compile for local host: `nix-build release.nix -A espressif-esp32-tools`
+* cross-compile for aarch64 (not working, yet): `nix-build release.nix -A espressif-esp32-tools --arg crossAarch64 true`
+* start nix-shell for aarch64 to debug build: `mkdir tmp; cd tmp; nix-shell ../release.nix -A espressif-esp32-tools --arg crossAarch64 true`, then `eval "$unpackPhase"`, `eval "$configurePhase"`, etc.

release.nix

-# usage:
-#   nix-build release.nix -A espressif-esp32-tools
-#   nix-build release.nix -A espressif-esp32-tools --arg crossAarch64 true
-#   mkdir tmp; cd tmp; nix-shell ../release.nix -A espressif-esp32-tools --arg crossAarch64 true
 let
   nixpkgs = import <nixpkgs>;
   overlay = import ./overlay.nix;