Add READMEs to individual crates

Signed-off-by: Nico Burns <nico@nicoburns.com>

malloc_size_of/README.md

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+# MallocSizeOf
+
+[![Build Status](https://github.com/servo/malloc_size_of/actions/workflows/main.yml/badge.svg)](https://github.com/servo/malloc_size_of/actions)
+[![Crates.io](https://img.shields.io/crates/v/malloc_size_of.svg)](https://crates.io/crates/malloc_size_of)
+[![Docs](https://docs.rs/malloc_size_of/badge.svg)](https://docs.rs/malloc_size_of)
+![Crates.io License](https://img.shields.io/crates/l/malloc_size_of)
+[![dependency status](https://deps.rs/repo/github/servo/malloc_size_of/status.svg)](https://deps.rs/repo/github/servo/malloc_size_of)
+
+A an allocator-agnostic crate for measuring the runtime size of a value
+including the size of any heap allocations that are owned by that value.
+
+This crate is used by both Servo and Firefox for memory usage calculation.
+
+## Features
+
+- It isn't bound to a particular heap allocator.
+- It provides traits for both "shallow" and "deep" measurement, which gives
+  flexibility in the cases where the traits can't be used.
+- It allows for measuring blocks even when only an interior pointer can be
+  obtained for heap allocations, e.g. `HashSet` and `HashMap`. (This relies
+  on the heap allocator having suitable support, which `jemalloc` has.)
+- It allows handling of types like `Rc` and `Arc` by providing traits that
+  are different to the ones for non-graph structures.

malloc_size_of_derive/README.md

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+# MallocSizeOfDerive
+
+A derive macro for the `MallocSizeOf` trait from the [malloc_size_of](https://crates.io/crates/malloc_size_of) crate.