Adds a new documentation page about usage in TypeScript projects

[aoberoi]

Summary

This page covers the most common questions we've seen when integrating this package into a TypeScript project.

fixes #612
fixes #615

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #644 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #644   +/-   ##
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  Coverage    91.9%   91.9%           
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  Files           7       7           
  Lines         482     482           
  Branches       90      90           
======================================
  Hits          443     443           
  Misses         22      22           
  Partials       17      17

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[RodneyU215]

Looks good!

[ronjouch]

@aoberoi @RodneyU215 are you sure about that? microsoft/TypeScript#26220 is still unmerged, and here on my machine with tsc 3.1.1, the correct flag is still esnext.asynciterable; compiling with es2018.asynciterable results into this error:

tsconfig.json:9:23 - error TS6046: Argument for '--lib' option must be: 'es5', 'es6', 'es2015', 'es7', 'es2016', 'es2017', 'es2018', 'esnext', 'dom', 'dom.iterable', 'webworker', 'webworker.importscripts', 'scripthost', 'es2015.core', 'es2015.collection', 'es2015.generator', 'es2015.iterable', 'es2015.promise', 'es2015.proxy', 'es2015.reflect', 'es2015.symbol', 'es2015.symbol.wellknown', 'es2016.array.include', 'es2017.object', 'es2017.sharedmemory', 'es2017.string', 'es2017.intl', 'es2017.typedarrays', 'es2018.intl', 'es2018.promise', 'es2018.regexp', 'esnext.array', 'esnext.symbol', 'esnext.asynciterable', 'esnext.intl'.

9     "lib": ["es2017", "es2018.asynciterable"],
                        ~~~~~~~~~~~~~~~~~~~~~~

node_modules/@slack/client/dist/util.d.ts:32:56 - error TS2304: Cannot find name 'AsyncIterable'.

[aoberoi]

Ah, okay it seems I should have done a little more homework. Thank you @ronjouch for checking my work.

microsoft/TypeScript#26218 was at one point set to milestone TS 3.1 - and so the information here was anticipated to be right, but it looks like the typescript team moved the goal post to TS 3.2. Not a problem, we'll just remove the part about es2018.asynciterable for now and hope we can catch up soon after that change lands.

[RodneyU215]

@ronjouch My over sight as well; I should have dug deeper. Thanks a lot for the feedback Ronan!