Update README.md with grammar tweaks

[brandondrew]

I hope this is helpful. Feel free to hit me up with questions if any changes are not clear, or if you think I misunderstood your intent.

Describe the change

Just touching up the grammar for clarity & readability.

Why are we doing this?

Greater readability means less cognitive overhead is required for understanding.

[piotrmurach]

Thank you for the suggestions. I like them bar the first one. Though I leave room to be convinced. I'm curious to hear what lead you to change write to compose?

[piotrmurach]

I'm not convinced by the 'compose'. By using the 'write', I wanted to emphasise that the slides are created in a text editor. It feels as though the 'compose' is taking the concept in another direction and further from the intent. However, I'm willing to be convinced that this is a good choice in this context. Though, in practice, I haven't heard anyone use the word 'compose' when working on slides.

[brandondrew]

Different verbs go well with different objects. "Write slides" may not be syntactically wrong, but it definitely sounds awkward. If you want to use the word "write", I would say something like "Create slides by simply writing in Markdown".