Also add .hhconfig to gitattributes export-ignore (#18)

Summary:
This means that when a release is installed, projects don't have
vendor/facebook/hsl-experiemental/.hhconfig, which can confuse IDE
integrations.
Pull Request resolved: #18

Differential Revision: D10105950

fbshipit-source-id: a5dee647dd1a3c1dc072d52e7c03e18eb45bf595

.gitattributes

@@ -1 +1,2 @@
 tests/ export-ignore
+.hhconfig export-ignore