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.md

@@ -1,3 +1,24 @@
 # Using the `wasmtime` CLI
 
-... more coming soon
+In addition to the embedding API which allows you to use Wasmtime as a
+library, the Wasmtime project also provies a `wasmtime` CLI tool to conveniently
+execute WebAssembly modules from the command line.
+
+This section will provide a guide to the `wasmtime` CLI and major functionality
+that it contains. In short, however, you can execute a WebAssembly file
+(actually doing work as part of the `start` function) like so:
+
+```sh
+$ wasmtime foo.wasm
+```
+
+Or similarly if you want to invoke a "start" function, such as with WASI
+modules, you can execute
+
+```sh
+$ wasmtime --invoke _start foo.wasm
+```
+
+For more information be sure to check out [how to install the
+CLI](cli-install.md) as well as [the list of options you can
+pass](cli-options.md).