Add "crate" to glossary.

Author: ehuss

src/glossary.md

@@ -50,6 +50,16 @@ Combinators are higher-order functions that apply only functions and
 earlier defined combinators to provide a result from its arguments.
 They can be used to manage control flow in a modular fashion.
 
+### Crate
+
+A crate is the unit of compilation and linking. There are different [types of
+crates], such as libraries or executables. Crates may link and refer to other
+library crates, called external crates. A crate has a self-contained tree of
+[modules], starting from an unnamed root module called the crate root. [Items]
+may be made visible to other crates by marking them as public in the crate
+root, including through [paths] of public modules.
+[More][crate].
+
 ### Dispatch
 
 Dispatch is the mechanism to determine which specific version of code is actually
@@ -274,6 +284,7 @@ example of an uninhabited type is the [never type] `!`, or an enum with no varia
 [associated item]: #associated-item
 [attributes]: attributes.md
 [*entity*]: names.md
+[crate]: crates-and-source-files.md
 [enums]: items/enumerations.md
 [fields]: expressions/field-expr.md
 [free item]: #free-item
@@ -291,6 +302,7 @@ example of an uninhabited type is the [never type] `!`, or an enum with no varia
 [lints]: attributes/diagnostics.md#lint-check-attributes
 [loop labels]: tokens.md#lifetimes-and-loop-labels
 [method]: items/associated-items.md#methods
+[modules]: items/modules.md
 [*Name resolution*]: names/name-resolution.md
 [*name*]: names.md
 [*namespace*]: names/namespaces.md
@@ -303,6 +315,7 @@ example of an uninhabited type is the [never type] `!`, or an enum with no varia
 [trait objects]: types/trait-object.md
 [traits]: items/traits.md
 [turbofish test]: https://github.com/rust-lang/rust/blob/master/src/test/ui/bastion-of-the-turbofish.rs
+[types of crates]: linkage.md
 [types]: types.md
 [undefined-behavior]: behavior-considered-undefined.md
 [unions]: items/unions.md