Remove Aragon CLI, document system deps

In order to prevent regular issues with the notoriously npm-issue-ridden
Aragon CLI module, this removes it as a direct dependency and requires
it to be installed as a system dependency.

Also adds Truffle as a system dependency to the docs (which was missing
before).

README.md

@@ -14,6 +14,8 @@ framework](http://truffleframework.com/) for some things.
 
 ### Installation
 
+#### App dependencies
+
 All requirements are defined in `package.json`.
 
     $ npm install
@@ -25,6 +27,13 @@ Each of the aragon apps are separate packages:
 
 You can use `npm run install-all` to install all app dependencies at once.
 
+#### Sytem dependencies
+
+Aragon CLI and Truffle need to be installed on your sytem as well:
+
+    npm install -g @aragon/cli
+    npm install -g truffle
+
 ### Local development chain
 
 For local development it is recommended to use