README: Instruct to install the binary

If you only build and not install, the binary won't be available under
`$GOPATH/bin/` as claimed by the following sentence.

Author: zeenix

README.md

@@ -77,15 +77,15 @@ You need also need libvirt-dev(el) package installed.
 ```console
 go get github.com/dmacvicar/terraform-provider-libvirt
 cd $GOPATH/src/github.com/dmacvicar/terraform-provider-libvirt
-make
+make install
 ```
 
 If you are using Go >= 1.11, you don't need to build inside GOPATH:
 
 ```
 export GO111MODULE=on
 export GOFLAGS=-mod=vendor
-make
+make install
 ```
 
 You will now find the binary at `$GOPATH/bin/terraform-provider-libvirt`.