Don't use prettier on Markdown files

Prettier does "opinionated" (read: annoying) things to unordered lists in
Markdown. See comments in file.

I would be far less angry about this if I could apply the fix in `.prettierrc`
instead of adding a whole new file to the top of the tree. If I run across
another prettier issue that ends in me writing a salty commit message, I'm
removing it from this project.

.prettierignore

@@ -0,0 +1,4 @@
+# Prettier does "opinionated" things to unordered lists in Markdown:
+# https://github.com/prettier/prettier/issues/5019
+# https://github.com/prettier/prettier/issues/5308
+**/*.md