Add instructions for bumping homebrew version (#2580)

Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

hack/brew-update.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+# This script assumes that 2 environment variables are defined outside of it:
+#   VELERO_VERSION - a full version version string, starting with v. example: v1.4.2
+#   HOMEBREW_GITHUB_API_TOKEN - the GitHub API token that the brew command will use to create a PR on the user's behalf.
+
+
+# Check if brew is found on the user's $PATH; exit if not.
+if [ -z $(which brew) ];
+then
+    echo "Homebrew must first be installed to use this script!"
+    exit 1
+fi
+
+# GitHub URL which contains the source code archive for the tagged release
+URL=https://github.com/vmware-tanzu/velero/archive/$VELERO_VERSION.tar.gz
+
+# Update brew so we're sure we have the latest Velero formula
+brew update
+
+# Download the Velero source tarball
+curl $URL --output velero-$VELERO_VERSION.tar.gz
+
+# Calculate the SHA 256 of the source tarball, since this is needed to bump the formula.
+SHA256=$(shasum -a 256 velero-$VELERO_VERSION.tar.gz)
+
+# Invoke brew's helper function, which will run all their tests and end up opening a browser with the resulting PR.
+brew bump-formula-pr --url $URL --sha256 $SHA256