Configures intersphinx. (#102)

* Refers back to Cartographer's system requirements.

docs/source/conf.py

@@ -45,6 +45,7 @@ import os
 extensions = [
     'sphinx.ext.todo',
     'sphinx.ext.mathjax',
+    'sphinx.ext.intersphinx',
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -266,3 +267,8 @@ texinfo_documents = [
 
 # If true, do not generate a @detailmenu in the "Top" node's menu.
 #texinfo_no_detailmenu = False
+
+intersphinx_mapping = {
+    'cartographer': ('https://google-cartographer.readthedocs.io/en/latest/',
+                     None)
+}

docs/source/index.rst

@@ -34,15 +34,14 @@ configurations. This project provides Cartographer's ROS integration.
 System Requirements
 ===================
 
-Although Cartographer ROS may run on other systems, it is confirmed to be
-working on systems that meet the following requirements:
+See Cartographer's :ref:`system requirements <cartographer:system-requirements>`.
+
+The following `ROS distributions`_ are currently supported:
 
-* 64-bit, modern CPU (e.g. 3rd generation i7)
-* 16 GB RAM
-* Ubuntu 14.04 (Trusty)
-* gcc version 4.8.4
 * ROS Indigo
 
+.. _ROS distributions: http://wiki.ros.org/Distributions
+
 Building & Installation
 =======================