add onlyIfDevelopment entry file for easier webpack config

[agilgur5]

• add documentation for this option and leave the previous installation
  docs as an alternative "custom installation"

I based the docs off of the previous commit that mentioned entry files, with a few modifications:

• Added a link to webpack entry config since it's a bit of an advanced topic
• Removed require.resolve as I don't believe it's necessary (I didn't need to use it in my webpack config, though I'm not sure if there's a circumstance in which it would be necessary)

Sorry, my formatter removed whitespace at the end of lines automatically, I can remove those from the commit if needed.

Having looked at the past docs commit, I can see that my usage is a bit different than the way you've used it / seen it. I didn't add a new entry (and therefore no new <script> tag), I added it to the array in my entry, i.e.

entry: {
  main: ['webpack-serve-overlay/onlyIfProduction', './main.js']
}

See the link above for full context in my boilerplate repo.
EDIT: I definitely think having it as a separate / new entry is the ideal usage as separate entrypoints / script tags ensures the overlay always shows up as its separate, isolated bundle will never fail to compile. In that case, excluding the overlay script tag from production HTML would also be ideal to have as little production impact as possible. Even with onlyIfProduction, webpack's bundling code will still create a tiny bundle of JS that does nothing (not optimal, but not particularly harmful either).

Related to #5

[G-Rath]

Looks pretty good.

Sorry it took a couple of days for me to get around to looking this over!

I'll give it a quick test with using entry, but aside from the two requested changes, it's looking pretty good!

[G-Rath]

Removed require.resolve as I don't believe it's necessary

I think this was based off the use of path.resolve with a lot of the other path-related things in webpack.

Having not had much experience with entry, I can't say if it's needed or not - it could very well have been the whole reason I was having problems.

I'm happy to defer to you for advise on the best way to handle the entry side of things 😃

[agilgur5]

Committed a new revision that should change it to onlyIfDevelopment and remove the formatter changes

[agilgur5]

Added a new commit with an addition to the docs regarding the hot reloading problem that occurs when this is the first entry