[WIP] Complete install instructions in the README.

- [ ] Write up Windows instructions.
- [ ] Write up homebrew instructions.
- [ ] Touch up various links.
- [ ] Fix internal links (see https://raw.githubusercontent.com/akshatpradhan/information-security-policy/policy_v1/README.md).
- [ ] Write up Ubuntu instructions.
- [ ] Write up UNIX instructions.
- [ ] Play with code blocks.
- [ ] Play with borders (`---`).
- [ ] Clean up external links (`[forum]`).