fix(README): overall improvements

[jeffmcaffer]

The doc is looking good. I found a few typos and wording issues, and have a few suggestions. Summary:

• the pluggability of octokit is exposed in section level bolded text (e.g., plugin: @octokit/plugin-rest-endpoint-methods) but the notion of plugins etc is not explained anywhere on the page. Here I suggest turning those references into "links to more doc" or some such and suggest that you add an explicit discussion about plugins somewhere and there, list all the plugins and summarize what they're for. Note also, some places the doc talks about "plugin" and others "standalone method". Not sure if those are the same or different things. In short, for readers of this page, simple is better IMO so remove concepts they don't need and have a separate section/doc that gives the more advanced user more info.
• Some of the changes are a bit pedantic but are trying to be brutally clear whether we're talking about GitHub or Octokit REST or request or ...
• I didn't go through the entire doc. I'm happy to review more but wanted to test the waters with these changes and see if it makes sense.
• There are a few places where I have questions. Will leave comments there once the PR is open

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View rendered README.md

[gr2m]

This is great thank you Jeff! I’m busy with other work right now but I’ll get back to your PR with a more thorough review soon

[gr2m]

Some feedback and further suggestions, please let me know what you think. Thanks again for helping with the documentation, I really appreciate it, and I really need more help with documentation in general

[jeffmcaffer]

That all looks good @gr2m. I updated the PR with your suggestions

[gr2m]

Thanks a lot Jeff!