Add formula back that prompts users to re-install.

After
d5ee047
it seems some users are stuck using the old formula. The author based
the change on
Homebrew/homebrew-cask@fdb4577
and they decided to add a caveat to the formula, warning users to remove
the formula.

brew-rmtree.rb

@@ -0,0 +1,21 @@
+class BrewRmtree < Formula
+    homepage "https://github.com/beeftornado/homebrew-rmtree"
+    url "https://github.com/beeftornado/homebrew-rmtree.git", :tag => "2.2.1"
+    version "2.2.1"
+
+    head "https://github.com/beeftornado/homebrew-rmtree.git"
+
+    def install
+    end
+
+    def caveats
+        <<~EOS.undent
+          You can uninstall this formula, as `brew tap beeftornado/brew-rmtree` is all that's
+          needed to install Rmtree and keep it up to date.
+        EOS
+      end
+
+    test do
+      system "brew", "rmtree", "--help"
+    end
+end