Add brewfile to bootstrap (github-linguist#6068)

Add Brewfile for bootstrapping deps on Mac

Co-authored-by: Colin Seymour <[email protected]>

.gitignore

@@ -13,3 +13,4 @@ test/fixtures/ace_modes.json
 *.so
 linguist-grammars*
 .venv
+Brewfile.lock.json

Brewfile

@@ -0,0 +1,4 @@
+brew "cmake"
+brew "pkg-config"
+brew "icu4c"
+cask "docker"

CONTRIBUTING.md

@@ -22,11 +22,7 @@ Linguist uses the [`charlock_holmes`](https://github.com/brianmario/charlock_hol
 [Docker](https://www.docker.com/) is also required when adding or updating grammars.
 These components have their own dependencies - `icu4c`, and `cmake` and `pkg-config` respectively - which you may need to install before you can install Linguist.
 
-For example, on macOS with [Homebrew](http://brew.sh/):
-```bash
-brew install cmake pkg-config icu4c
-brew install --cask docker
-```
+On macOS with [Homebrew](http://brew.sh/) the instructions below under Getting started will install these dependencies for you.
 
 On Ubuntu:
 ```bash

script/bootstrap

@@ -4,6 +4,13 @@ set -e
 
 cd "$(dirname "$0")/.."
 
+if [ -f "Brewfile" ] && [ "$(uname -s)" = "Darwin" ]; then
+  brew bundle check >/dev/null 2>&1  || {
+    echo "==> Installing Homebrew dependencies…"
+    brew bundle
+  }
+fi
+
 bundle config --local path vendor/gems
 
 bundle check > /dev/null 2>&1 || bundle install