Update install.md

I found this issue (https://github.com/derekparker/delve/issues/514) where user alexbrainman gave a very helpful answer. Adding this to the official install-guide might help a lot of users.

Documentation/installation/windows/install.md

@@ -6,4 +6,6 @@ Please use the standard `go get` command to build and install Delve on Windows.
 go get -u github.com/derekparker/delve/cmd/dlv
 ```
 
+Also, if not already set, you have to add the %GOPATH%\bin directory to your PATH variable.
+
 Note: If you are using Go 1.5 you must set `GO15VENDOREXPERIMENT=1` before continuing. The `GO15VENDOREXPERIMENT` env var simply opts into the [Go 1.5 Vendor Experiment](https://docs.google.com/document/d/1Bz5-UB7g2uPBdOx-rw5t9MxJwkfpx90cqG9AFL0JAYo/).