Docs: installation instructions re: unmet peer dependencies

[jaredbeck]

See discussion in #1078

[jaredbeck]

For example, a clean install of webpacker today (2018-05-11) according to the previous instructions results in the following:

warning "@rails/webpacker > [email protected]" has unmet peer dependency "caniuse-lite@^1.0.30000697".
warning " > [email protected]" has unmet peer dependency "webpack@^2.2.0 || ^3.0.0".
warning "webpack-dev-server > [email protected]" has unmet peer dependency "webpack@^1.0.0 || ^2.0.0 || ^3.0.0".

yarn upgrade fixes these warnings, so it might be helpful to include in the instructions

[jaredbeck]

One month ping, please review

[chriscarpenter12]

Can confirm. I just started a new project and couldn't get webpack-dev-server to start. Ran yarn upgrade and it worked.

[jaredbeck]

Three month ping. This is docs-only, should be a quick review, thanks.

[gauravtiwari]

Thanks @jaredbeck and apologies for delay :)