DOCS: Add --single-branch to prevent fetching of all branches.

Author: cafferata

documentation/test-a-branch.md

@@ -12,7 +12,7 @@ Using Docker assures you're using the latest version of Go and doesn't require y
 
 ```shell
 docker run -it -v /THE/PATH:/dns golang
-git clone -b INSERT_BRANCH_HERE https://github.com/StackExchange/dnscontrol.git
+git clone -b INSERT_BRANCH_HERE --single-branch https://github.com/StackExchange/dnscontrol.git
 cd dnscontrol
 go install
 ```
@@ -42,7 +42,7 @@ Step 1: Install Go
 Step 2: Check out the software
 
 ```shell
-git clone -b INSERT_BRANCH_HERE https://github.com/StackExchange/dnscontrol.git
+git clone -b INSERT_BRANCH_HERE --single-branch https://github.com/StackExchange/dnscontrol.git
 cd dnscontrol
 go install
 ```