Small language changes in `README.md`

[BelKed]

• 5660e91 Add a link to the list of directories – It's now easier to understand what "ActivityWatch directory" means
• 8101e0f Type "ActivityWatch" correctly – It should be typed "ActivityWatch"
• 9fefe56 Add a full stop after each sentence – For consistency
• 704b5c8 Small language changes – The sentence in the Testing section sounded a little weird to me, so I changed it...
• ea077d3 Add backticks before and after some terms – It's now clearer I guess...
• 3aad9b7 Change the Contribute section a bit – Again, I found it strange to thank first and then point to DEV_SETUP.md
• f27d10c Use gerund verbs in headings – I think personal preferences, but now it seems better to me
• e4ee993 Correct the last step number – It should be 6, not 5

[Alwinator]

@BelKed Thank you very much for your contribution! ;)