Add information about haskell#887

Updates the README for haskell#887

Author: lukel97

README.md

@@ -484,6 +484,8 @@ $ cd haskell-ide-engine
 $ stack --stack-yaml=<stack.yaml you used to build hie> exec hoogle generate
 ```
 
+Or you can set the enviornment `HIE_HOOGLE_DATABASE` to specify a specific database.
+
 ## Contributing
 
 ### Planned Features