readme: add protocol to URL
if copy-pasted or typed verbatim the clone would fail.
@@ -37,7 +37,7 @@ First, checkout and install the operator-sdk CLI:
```sh
$ mkdir -p $GOPATH/src/github.com/operator-framework
$ cd $GOPATH/src/github.com/operator-framework
-$ git clone github.com/operator-framework/operator-sdk
+$ git clone https://github.com/operator-framework/operator-sdk
$ cd operator-sdk
$ git checkout master
$ make dep
