Add .yamllint and disable some annoying recurring warnings on every…

… pr (llvm#3224)

.yamllint.yml

@@ -0,0 +1,22 @@
+---
+
+extends: default
+
+rules:
+  # These do not appear to be conventional in GitHub actions.
+  document-end:
+    present: false
+  document-start:
+    present: false
+  # GitHub actions use "on" for triggers.
+  truthy: disable
+  # We have lots of long strings and command lines.
+  line-length: disable
+  comments:
+    # Formatters may do this (e.g. Prettier does) and it seems like the most
+    # trivial thing to get a failing check for.
+    min-spaces-from-content: 1
+  # This is not a useful check, especially when disabling entire blocks.
+  comments-indentation: disable
+
+ignore: /third_party/*