Add html styleguide with initial rule

First rule:
Since quotes are sometimes required when specifying HTML attribute
values, quote consistently to reduce thought and diffs.

Author: ABaldwinHunter

README.md

@@ -26,6 +26,7 @@ course of your current work. Do not change code *only* to fix style.
 This repository includes written guides & relevant linter configuration for the
 following languages:
 
-- [Ruby](ruby/README.md)
-- [JavaScript](javascript/README.md)
 - [Haskell](haskell/README.md)
+- [HTML](markup/README.md)
+- [JavaScript](javascript/README.md)
+- [Ruby](ruby/README.md)

html/README.md

@@ -0,0 +1,15 @@
+# HTML
+
+- Always use quotes when specifying attribute values.
+
+  ```html
+  <!-- Good -->
+    <td class="foo"></td>
+    <td class="foo bar"></td>
+
+  <!-- Bad -->
+    <td class=foo></td>
+  ```
+
+  Reasoning: since quotes are required in some situations, quote consistently
+  to minimize thought points + diffs.