Add ccache to drake documentation

[jwnimmer-tri]

Previously removed in #2511. Requested to return by Russ in #3509.

[jwnimmer-tri]

Consider asking for ccache -o run_second_cpp=true during one-time setup, instead of CCACHE_CPP2 like we used to recommend.

Still recommend the magic path addition (not a CMake launcher).

#2640 would be an opportunity to add in ccache instructions directly into a "developer-only" workflow.

Alternatively, we could have the setup/ubuntu/16.04/install_prereqs.sh do the right thing (and add OS X setup to match).

[jwnimmer-tri]

FWIW existing private docs:

Ubuntu

    sudo apt-get install ccache
    Add the following to your .bashrc: 
        export PATH="/usr/lib/ccache:$PATH"
        export CCACHE_CPP2=yes

OS X

    brew install ccache
    Add the following to your .bashrc: 
        export PATH="/usr/local/opt/ccache/libexec:$PATH"
        export CCACHE_CPP2=yes
    Make sure your .bash_profile is running your .bashrc:
        if [ -f ~/.bashrc ]; then . ~/.bashrc; fi 

The apt / brew should move into the project-wide list of apt / brew (along with all other deps).
The ccache -o run_second_cpp=true replaces CCACHE_CPP2.

The PATH edit will still have to happen somehow. Consider combining with other PATH edits as already included in the setup docs. Maybe it is time to have an addpath_drake.sh helper.

[jwnimmer-tri]

Nominating david-german to track this, though I think anyone should feel free to take over this improvement.

[jwnimmer-tri]

The CCACHE_CPP2 flea has bitten us a few times recently; though in newest ccache it is finally the default.

Though with #3129 we shouldn't really use ccache anyway, other than #4464.

[jwnimmer-tri]

I think this is no longer relevant given #3129's progress.