Spice up the project README, fill out more docs (#1010)

* Spice up the project README, fill out more docs

This is an attempt to spruce up wasmtime's README.md file as well as
fill out more of the missing documentation in the `docs/` folder.
There's still a long way to go but I hoped here to mostly move around
existing information and add new information. As always happy to have
feedback!

* Tweak CLI wording

* Remove no-longer relevant clause

* Update sandboxing docs

* Handle comments

docs/cli-options.md

@@ -1,3 +1,82 @@
 # CLI Options for `wasmtime`
 
-... more coming soon
+The `wasmtime` CLI is organized into a few subcommands. If no subcommand is
+provided it'll assume `run`, which is to execute a wasm file. The subcommands
+supported by `wasmtime` are:
+
+## `help`
+
+This is a general subcommand used to print help information to the terminal. You
+can execute any number of the following:
+
+```sh
+$ wasmtime help
+$ wasmtime --help
+$ wasmtime -h
+$ wasmtime help run
+$ wasmtime run -h
+```
+
+When in doubt, try running the `help` command to learn more about functionality!
+
+## `run`
+
+This is the `wasmtime` CLI's main subcommand, and it's also the default if no
+other subcommand is provided. The `run` command will execute a WebAssembly
+module. This means that the module will be compiled to native code,
+instantiated, and then optionally have an export executed.
+
+The `wasmtime` CLI will automatically hook up any WASI-related imported
+functionality, but at this time if your module imports anything else it will
+fail instantiation.
+
+The `run` command takes one positional argument which is the name of the module
+to run:
+
+```sh
+$ wasmtime run foo.wasm
+$ wasmtime foo.wasm
+```
+
+Note that the `wasmtime` CLI can take both a binary WebAssembly file (`*.wasm`)
+as well as the text format for WebAssembly (`*.wat`):
+
+```sh
+$ wasmtime foo.wat
+```
+
+## `wast`
+
+The `wast` command executes a `*.wast` file which is the test format for the
+official WebAssembly spec test suite. This subcommand will execute the script
+file which has a number of directives supported to instantiate modules, link
+tests, etc.
+
+Executing this looks like:
+
+```sh
+$ wasmtime wast foo.wast
+```
+
+## `config`
+
+This subcomand is used to control and edit local Wasmtime configuration
+settings. The primary purpose of this currently is to configure [how Wasmtime's
+code caching works](./cli-cache.md). You can create a new configuration file for
+you to edit with:
+
+```sh
+$ wasmtime config new
+```
+
+And that'll print out the path to the file you can edit.
+
+## `wasm2obj`
+
+This is an experimental subcommand to compile a WebAssembly module to native
+code. Work for this is still heavily under development, but you can execute this
+with:
+
+```sh
+$ wasmtime wasm2obj foo.wasm foo.o
+```