Change recommendation to install truffle locally instead of globally

[benjamincburns]

Installing truffle locally to a project helps to ensure a consistent experience across that project's team and (most importantly) their CI builds. Many mature projects already do this, and we should make it our official recommendation.

It's an open question (please do debate in the comments) as to whether we should recommend against a global install.

On discussion of this issue with @gnidan, he suggested that perhaps we might (on release of Truffle v6) want to follow webpack's model where there's a globally-installable CLI package and a locally-installable library that's used by that package.

Another alternative might be to make it so that when the globally-installed truffle is executed on a project that has a local truffle install, the global install does a process.exec out to the local install.

If we opt for either of these changes we'll need to be sure to raise an issue for them in the truffle project.