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/introduction.md

@@ -1,3 +1,29 @@
 # Introduction
 
-... more coming soon
+[Wasmtime][github] is a [Bytecode Alliance][BA] project that is a standalone
+wasm-only optimizing runtime for [WebAssembly] and [WASI]. It runs WebAssembly
+code [outside of the Web], and can be used both as a command-line utility or as
+a library embedded in a larger application.
+
+Wasmtime strives to be a highly configurable and embeddable runtime to run on
+any scale of application. Many features are still under development so if you
+have a question don't hesitate to [file an issue][issue].
+
+This guide is intended to server a number of purposes and within you'll find:
+
+* [How to create simple wasm modules](tutorial-create-hello-world.md)
+* [How to use Wasmtime from a number of languages](lang.md)
+* [How to use install and use the `wasmtime` CLI](cli.md)
+* Information about [stability](stability.md) and [security](security.md) in
+  Wasmtime.
+
+... and more! The source for this guide [lives on
+GitHub](https://github.com/bytecodealliance/wasmtime/tree/master/docs) and
+contributions are welcome!
+
+[github]: https://github.com/bytecodealliance/wasmtime
+[BA]: https://bytecodealliance.org/
+[WebAssembly]: https://webassembly.org/
+[WASI]: https://wasi.dev
+[outside of the Web]: https://webassembly.org/docs/non-web/
+[issue]: https://github.com/bytecodealliance/wasmtime/issues/new