Add more content to cranelift-entity's README.md. (#515)

* Add more content to cranelift-entity's README.md.

Summarize what cranelift-entity provides, and how it differs from
similar systems such as slotmap, which was recently highlighted in the
RustConf 2018 Closing Keynote.

lib/entity/README.md

@@ -1,2 +1,40 @@
 This crate contains array-based data structures used by the core Cranelift code
 generator which use densely numbered entity references as mapping keys.
+
+One major difference between this crate and crates like [slotmap], [slab],
+and [generational-arena] is that this crate currently provides no way to delete
+entities. This limits its use to situations where deleting isn't important,
+however this also makes it more efficient, because it doesn't need extra
+bookkeeping state to reuse the storage for deleted objects, or to ensure that
+new objects always have unique keys (eg. slotmap's and generational-arena's
+versioning).
+
+Another major difference is that this crate protects against using a key from
+one map to access an element in another. Where `SlotMap`, `Slab`, and `Arena`
+have a value type parameter, `PrimaryMap` has a key type parameter and a value
+type parameter. The crate also provides the `entity_impl` macro which makes it
+easy to declare new unique types for use as keys. Any attempt to use a key in
+a map it's not intended for is diagnosed with a type error.
+
+Another is that this crate has two core map types, `PrimaryMap` and
+`EntityMap`, which serve complementary purposes. A `PrimaryMap` creates its
+own keys when elements are inserted, while an `EntityMap` reuses the keys
+values of a `PrimaryMap`, conceptually storing additional data in the same
+index space. `EntityMap`'s values must implement `Default` and all elements
+in an `EntityMap` initially have the value of `default()`.
+
+A common way to implement `Default` is to wrap a type in `Option`, however
+this crate also provides the `PackedOption` utility which can use less memory
+in some cases.
+
+Additional utilities provided by this crate include:
+ - `EntityList`, for allocating many small arrays (such as instruction operand
+    lists in a compiler code generator).
+ - `SparseMap`: an alternative to `EntityMap` which can use less memory
+   in some situations.
+ - `EntitySet`: a specialized form of `EntityMap` using a bitvector to
+   record which entities are members of the set.
+
+[slotmap]: https://crates.io/crates/slotmap
+[slab]: https://crates.io/crates/slab
+[generational-arena]: https://crates.io/crates/generational-arena