Clarify note about loading polyfills only once

[darahak]

Reading the note, I wasn't sure if it could cause errors or not, and I had to look around for a little while.
Now it will say explicitly that loading babel-polyfill more than once will throw an error.

[hzoo]

I did put

Only use require("babel-polyfill"); once in your whole app

But if this makes it clearer, we can do that. I can remove multiple imports if it's in the same file but if you actually import polyfill multiple times then yes it will error. Actually now that I think about it, using useBuiltIns won't import babel-polyfill so that error message won't be hit

[existentialism]

Yeah, I'd prefer if we just combine the statements, something like:

NOTE: Only use require("babel-polyfill"); once in your whole app. Multiple imports or requires of babel-polyfill will throw an error. We recommend creating a single entry file that only contains the require statement.

[codecov-io]

Codecov Report

Merging #282 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #282   +/-   ##
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  Coverage   93.03%   93.03%           
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  Files           4        4           
  Lines         201      201           
  Branches       59       59           
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  Hits          187      187           
  Misses          9        9           
  Partials        5        5

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[darahak]

@existentialism Your proposal has better phrasing. I would like to keep the rationale though.

[existentialism]

Sure:

NOTE: Only use require("babel-polyfill"); once in your whole app. Multiple imports or requires of babel-polyfill will throw an error since it can cause global collisions and other issues that are hard to trace. We recommend creating a single entry file that only contains the require statement.

(Feel free to modify, just wanted to provide a quick example)

[darahak]

Thanks.
I updated this with your suggestion.

[existentialism]

@darahak thanks!