Add an Overview doc.

Overview gives a high-level view of the crates in the Tock 2.0 libtock-rs. Not all of these crates have been merged into this repository yet, but all of them are a work in progress.

doc/Overview.md

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+Overview
+======
+
+Note: This describes the layout of the Tock 2.0 `libtock-rs` crates, not the
+Tock 1.0 crates. Not all of the Tock 2.0 crates have been merged into this
+repository yet.
+
+This document gives an overview of the crates in this repository, and is
+intended to be useful to `libtock-rs` newcomers.
+
+## `libtock`
+
+Note: `libtock` is currently called `libtock2`, as `libtock` is already a Tock
+1.0 crate.
+
+`libtock` provides the default `libtock-rs` experience. It re-exports all of the
+drivers `libtock-rs` provides, and provides usable defaults for panic handling
+and memory allocation. It should be easy to build a Tock application that only
+has one direct dependency: `libtock`.
+
+In order to be easy to use, `libtock` lacks some functionality that the other
+`libtock-rs` crates provide. In particular, you should depend on the other
+crates directly if you want any of the following:
+
+* The ability to unit test your code.
+* The ability to customize panic handling.
+* The ability to run without dynamic memory allocation or with an alternative
+  memory allocator.
+
+## Naming convention note
+
+Although these crates have yet to be uploaded to crates.io, they likely will be
+uploaded in the future. Therefore, to avoid name collisions, most crates in this
+repository have a name that begins with `libtock`. Crates that are only intended
+for internal use (e.g. `syscall_tests`, which tests code internal to
+`libtock_platform`) do not have the `libtock_` prefix.
+
+The directory names of `libtock_` crates do not contain the `libtock_` prefix.
+
+## Core abstractions: `libtock_platform`
+
+In order to unit test `libtock-rs` code, we need a way to run `libtock-rs` on
+our development machines (and in CI). Ironically, that means most crates in
+`libtock-rs` are platform independent. `libtock_platform` provides the tools
+that allow code to run in both a unit test environment and in real Tock apps. It
+consists primarily of the `Syscalls` trait and supporting machinery.
+
+## Syscall implementations: `libtock_runtime` and `libtock_unittest`
+
+In order to `libtock-rs` code, you need a `libtock_platform::Syscalls`
+implementation. Multiple implementations exist that work in different
+environments:
+
+* `libtock_runtime` provides a syscall interface that uses a real Tock kernel.
+  This is the crate to use in Tock process binaries.
+* `libtock_unittest` provides a fake kernel for use in unit tests.
+
+In addition, `libtock_runtime` provides the linker script and Rust runtime
+needed to start a Tock process binary. `libtock_unittest` relies on `std` to
+provide a runtime.
+
+## Panic handler crates
+
+Each Rust binary must have exactly one panic handler (note that `std` provides a
+panic handler for binaries that depend on it). The following crates provide a
+`#[panic_handler]` implementation for Tock process binaries:
+
+* `libtock_panic_debug` provides useful diagnostics in the event of a panic, at
+  the expense of code size. This is the panic handler used by `libtock`.
+
+## Driver crates
+
+Driver crates provide interfaces to specific Tock APIs:
+
+| Crate                   | Tock API        |
+|-------------------------|-----------------|
+|`libtock_console`        |[Console]        |
+|`libtock_low_level_debug`|[Low-Level Debug]|
+
+[Console]: https://github.com/tock/tock/blob/master/doc/syscalls/00001_console.md
+[Low-Level Debug]: https://github.com/tock/tock/blob/master/doc/syscalls/00008_low_level_debug.md