Document Homebrew install, reorganize README, and check it against the binary (#62)

* Document Homebrew install and reorganize README install section

* Say which install methods cover which platforms

* Restore the sixth target and PowerShell note lost in the rewrite

* Reorganize README around what a reader does first

* Check README's commands against the binary

The README listed four API groups (rasterarrays, static-images, static-tiles, tilequery) the binary no longer has, and never mentioned doctor, usage or the real static group. Nothing read the README, so nobody noticed.

* Fix README's command list and cut detail docs/commands.md already has

Author: zmofei

CONTRIBUTING.md

@@ -39,6 +39,10 @@ and not a style note.
 and a fake server, so the suite runs offline and on a machine that has never
 logged in.
 
+`scripts/test-install.sh` and `scripts/test-install.ps1` exercise the
+installers end to end without touching the network, and
+`scripts/test-completion.sh` does the same for the completion scripts.
+
 Four rules the compiler holds rather than a reviewer, declared in
 `Cargo.toml` with the reasoning beside each: no `unsafe`, no `println!`
 (stdout belongs to `output::emit`, the single place `--output` is honored),