yle-dl: add caveats on need for mcrypt

Closes Homebrew/homebrew#48489.
Closes Homebrew/homebrew#48650.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/yle-dl.rb

@@ -15,6 +15,11 @@ class YleDl < Formula
 
   depends_on "rtmpdump"
   depends_on :python if MacOS.version <= :snow_leopard
+  depends_on "homebrew/php/php70-mcrypt" => :optional
+  depends_on "homebrew/php/php56-mcrypt" => :optional
+  depends_on "homebrew/php/php55-mcrypt" => :optional
+  depends_on "homebrew/php/php54-mcrypt" => :optional
+  depends_on "homebrew/php/php53-mcrypt" => :optional
 
   resource "AdobeHDS.php" do
     # NOTE: yle-dl always installs the HEAD version of AdobeHDS.php. We use a specific commit.
@@ -43,6 +48,17 @@ class YleDl < Formula
     bin.env_script_all_files(libexec/"bin", :PYTHONPATH => ENV["PYTHONPATH"])
   end
 
+  def caveats; <<-EOS.undent
+    yle-dl requires the mcrypt PHP module which you can either install manually or install
+    mcrypt from the PHP homebrew tap. You can also install yle-dl with one of optional dependencies
+    which to automatically tap the php tap and download mcrypt module for you.
+
+      brew info yle-dl
+
+    for further info.
+    EOS
+  end
+
   test do
     assert_equal "Yle - Sinun tarinasi: 3 minuuttia-2012-05-30T10:51:00+03:00\n",
                  shell_output("#{bin}/yle-dl --showtitle http://areena.yle.fi/1-1570236")