[WIP] Complete install instructions in the README.

- [x] Write up Windows instructions.
- [x] Write up homebrew instructions.
- [x] Touch up various links.
- [x] Fix internal links (see https://raw.githubusercontent.com/akshatpradhan/information-security-policy/policy_v1/README.md).
- [x] Write up Ubuntu instructions.
- [x] Write up UNIX instructions.
- [x] Play with code blocks.
- [x] Play with borders (`---`).
- [x] Clean up external links (`[forum]`).
- [x] Figure out `lib/`, `x86_64-linux-gnu`, (DY)LD_LIBRARY_PATH issue.
- [x] Should we include instructions about SimTK_INSTALL_DIR?
- [x] Homebrew seems not to be working on @apoorvar's Mac.