Dart install page for Linux should also instruct users add the Dart tools to their path.

[DaveShuckerow]

I wanted to try setting up a basic Angular app and could not find a link on how to add the appropriate packages to get pub running my code locally in a browser from the page on getting started with Dart webdev.

Eventually I found the page on Getting started with pub and noticed that I hadn't added pub to my path.

I would not have encountered this problem if I had known that the install phase for Dart had not added it to my path.

We can avoid this problem by adding a line on how to do this to the initial page for Installing Dart on Linux.

[kwalrath]

We should add this to all the installation pages.