Revised readme for clarification

[gness1804]

I added some clarification to the readme.md file. When I was trying to activate the spell check for html files, I found that the documentation could use some clarification by breaking down some of the steps for adding language scopes and by tweaking some of the language. This edit should make it a bit easier for users to customize spell check.

[thomasjo]

I think this looks good. @atom/feedback What do you think? 👀

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@gness1804 For future reference, consider adding the [ci skip] marker to the commit description instead of the commit subject line. Just makes it more pleasant to actually read the log 🙇

Minor change to readme

[ci skip]

[winstliu]

Looks good to me as well.

[gness1804]

@thomasjo Cool, I'll keep that in mind. Thanks for the tip!