Add .markdown-link-check.json file

and ignore URL with bad SSL cert: www.conventionalcommits.org

This commit provides less conflicts with upstream whenever we decide to
rebase in the future b/c this file doesn't exist in upstream.

.markdown-link-check.json

@@ -0,0 +1,7 @@
+{
+  "ignorePatterns": [
+    {
+      "pattern": "^https://www.conventionalcommits.org"
+    }
+  ]
+}