Docs: clarify 3 items

[Cerchie]

This worked on the first try! Nifty. 😊

Had to fill in the blanks in a couple of places in the README while I read it, so I added the following clarifications:

• make clear that you have to run npm install before npm run dev
• make clear that 'hard-coding the markdown' means 'adding uri-encoded markdown to the url'
• make clear that the GET request publishes your cached post

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[cassidoo]

Amazing, thank you!!