Add clarity to README, fix broken image link, fix typos

[beth]

I was just running through the demo myself, and made a bunch of minor readme fixes.

Listed roughly in order of occurrence -

1. Add note about needing to create organization and space after signing up for bluemix.
2. Switch order of login/connect to bluemix from connect, then login => login, then connect.
3. Add a step to Getting Started to direct human to go to URL (I know it's already implied, but humans!).
4. Make it explicit that "make a .env file" means "make a file named .env".
5. Make explicit that installing node will also install npm.
6. Format npm start and npm install to indicate terminal command.
7. Indicate optional for all optional environment variables.
8. Rephrase sentence about OVERRIDE_CLASSIFIER_ID to match sentence structure of other env variable descriptions.
9. Fix broken tooltip image link.
10. Fix typo of localy => locally.
11. Remove extraneous spaces.
12. Make periods at end of lines consistent.
13. Turn node and npm links into reference links.

[kognate]

Looks good. Thank you for doing this.