Add YAML file linter (#6896)

This adds a corresponding configuration file `.yamllint.yaml` so that people can run `yamllint` to check the YAML files for syntax errors and conformance to style guidelines.

Author: mhucka

.yamllint.yaml

@@ -0,0 +1,14 @@
+# Summary: yamllint configuration for Cirq.
+# See https://yamllint.readthedocs.io/ for info about configuration options.
+
+rules:
+  line-length:
+    # YAML files (especially GitHub Actions workflows) tend to end up with
+    # long lines. The default of 80 is pretty limiting, and besides, in Python
+    # code linting, we set line lengths to 100. May as well follow suit here.
+    max: 100
+    # Another common occurrence in YAML files is long URLs. The next two
+    # settings are not specific to URLs, but help. It saves developer time by
+    # not requiring comment directives to disable warnings at every occurrence.
+    allow-non-breakable-words: true
+    allow-non-breakable-inline-mappings: true