readme: add logo (#322)

per @jvehent logo is licensed for public use

README.md

@@ -1,5 +1,7 @@
 # Frost
 
+![frost snowman logo](docs/frost-snowman-logo.png?raw=true)
+
 Clients and [pytest](https://docs.pytest.org/en/latest/index.html)
 tests for checking that third party services the @foxsec team uses are
 configured correctly.