add markdown config file

.markdownlint.yml

+##
+# Rules for markdownlint.
+#
+# See https://github.com/DavidAnson/markdownlint
+##
+
+default: true
+
+# MD013/line-length - Line length
+MD013:
+  # Number of characters
+  line_length: 100
+  # Number of characters for headings
+  heading_line_length: 100
+  # Number of characters for code blocks
+  code_block_line_length: 100