Update installation.md (#6415)

It might seem redundant, but I followed this whole tutorial not knowing what <path-to-tensorflow> meant, and tried to install into the actual "tensorflow" package inside of my python installation.

research/object_detection/g3doc/installation.md

@@ -53,6 +53,14 @@ pip install --user matplotlib
 Protobuf 3+ versions for you and some users have issues when using 3.5.
 If that is your case, try the [manual](#Manual-protobuf-compiler-installation-and-usage) installation.
 
+## Download the tensorflow/models repository
+
+```bash
+git clone https://github.com/tensorflow/models.git
+```
+
+To use this library, you need to download this repository, whenever it says `<path-to-tensorflow>` it will be referring to the folder that you downloaded this repository into.
+
 ## COCO API installation
 
 Download the