Configure flake8-rst-docstrings for common Sphinx roles

[cjolowicz]

diff --git a/.flake8 b/.flake8
index 528cb40..0f46431 100644
--- a/.flake8
+++ b/.flake8
@@ -5,3 +5,5 @@ max-line-length = 80
 max-complexity = 10
 docstring-convention = google
 per-file-ignores = tests/*:S101
+rst-roles = class,const,func,meth,mod,ref
+rst-directives = deprecated