repo: update readme

[buck54321]

• Expand overview. Remove overview header. The description can just go below the title.
• Move build status to the bottom, or at least below the fold. It is not important for most visitors.
• Add section about current budget and objectives. Replaces the roadmap section.
• Add contributing / development section. Can be a link to a separate file.
• Add some small code examples. We moved the bigger examples to various scripts in create example scripts #6, but a couple smaller examples to demonstrate simple tasks would be great.

[teknico]

Maybe add a "Development" section too, unless it's the same as the "Contributing" one? It should mention pytest, black, flake8 and the .github/workflows/python.yml file. (I forgot to add it as part of #27, sorry.)

[teknico]

I'm working on the README as part of #59. There will be three README files: the current one at the top of the repo and one for each of the decred and tinywallet packages. The decred one will include the text currently in pydecred/README.md.

I'm not sure we should de-emphasize the build status: it's significant to project users as an indication of health and ongoing maintenance, and the placement at the top of the main README is common and expected on Github.

I'll add a CONTRIBUTING.md file with a "Development" section, and link to it from the main README.

I'll leave the other items to you.