Add comments to markdownlint configuration

Now that this is a YAML file we can add comments explaining the rule
modifications we use. This will make it easier to edit or expand in the future.

.mdl_config.yaml

@@ -1,24 +1,40 @@
 ---
 
+# Default state for all rules
 default: true
 
+# MD003/heading-style/header-style - Heading style
 MD003:
+  # Enforce the ATX-closed style of header
   style: "atx_closed"
 
+# MD004/ul-style - Unordered list style
 MD004:
+  # Enforce dashes for unordered lists
   style: "dash"
 
+# MD013/line-length - Line length
 MD013:
+  # Do not enforce for code blocks
   code_blocks: false
+  # Do not enforce for tables
   tables: false
 
+# MD024/no-duplicate-heading/no-duplicate-header - Multiple headings with the
+# same content
 MD024:
+  # Allow headers with the same content as long as they are not in the same
+  # parent heading
   allow_different_nesting: true
 
+# MD029/ol-prefix - Ordered list item prefix
 MD029:
+  # Enforce the `1.` style for ordered lists
   style: "one"
 
+# MD033/no-inline-html - Inline HTML
 MD033:
+  # The h1 and img elements are allowed to permit header images
   allowed_elements:
     - h1
     - img