docs: add editorconfig for markdown

I was trying to fix a link in in a document and couldn't get github, my editor or my local git to perserve the mixed line endings in the file. Instead of doing that I figued I could help fix it.

[Editorconfig](https://editorconfig.org/#) is nativly supported by most popular editors and keeps formatting issues like this away. I've set it up for markdown files using the most commonly used line endings.

.editorconfig

@@ -0,0 +1,6 @@
+root = true
+
+[*.md]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true