Added new Haml cheatsheet. Includes Ruby injection with some examples of variable declaration and output. (#736)

haml.md

@@ -0,0 +1,53 @@
+---
+title: Haml
+category: Markup
+layout: 2017/sheet
+prism_languages: [haml]
+---
+
+### Doctype
+
+```haml
+!!! 5
+```
+
+### Tags
+
+```haml
+%html
+  %head
+    %title
+  %body
+    %h1 Hello World
+    %br/
+```
+
+### Classes and ID's
+
+```haml
+%p.class-example
+.no-tag-defaults-to-div
+%div#butItCanBeIncluded
+```
+
+### Inline Attributes
+
+Either hash syntax works
+
+```haml
+%meta{ name: "viewport", content: "width=device-width, initial-scale=1.0" }
+%input{ :type => "text", :required => true }
+```
+
+### Ruby
+
+```haml
+-# This is a comment
+-# Anything starting with a hyphen signals to Haml that Ruby is coming
+- @arr = [1, 2, 3]
+- @str = "test"
+-# Equal signals output
+= render partial: "shared/header"
+= yield
+= link_to page_url
+```