New markdown lint JSON file

Defines certain rules for markdown linting such as the allowed usage of
`<br>` elements, etc.

.markdownlint.json

@@ -0,0 +1,10 @@
+{
+  "default": true,
+  "line-length": {
+    "line_length": 100,
+    "tables": false
+  },
+  "no-inline-html": {
+    "allowed_elements": ["br"]
+  }
+}