Really run yamllint (it wasn't checking any rule)

Fix the yamllint config in `.yamllint` to "extend" the default rule.
Previously, it didn't extend anything and only disabled a rule, which
means no rule at all were checked.

Also disable some rules in this file, because they report many errors in
the Sigma code base.

In the future, I suggest fixing these errors and re-enabling standard
rules like `trailing-spaces` or `indentation`.

Fixes #220.

.yamllint

@@ -1,4 +1,12 @@
 ---
 # https://yamllint.readthedocs.io/en/latest/configuration.html
+extends: default
 rules:
+  comments: disable
+  comments-indentation: disable
   document-start: disable
+  empty-lines: {max: 2, max-start: 2, max-end: 2}
+  indentation: disable
+  line-length: disable
+  new-line-at-end-of-file: disable
+  trailing-spaces: disable