Add explanations of cross-version crate compatibility to README

Author: dhardy

README.md

@@ -38,6 +38,16 @@ To get started using Rand, see [The Book](https://rust-random.github.io/book).
 
 ## Versions
 
+Rand libs have inter-dependencies and make use of the
+[semver trick](https://github.com/dtolnay/semver-trick/) in order to make traits
+(particularly `RngCore`) compatible across crate versions. This means that a
+few crate releases (`rand_core` versions `0.2.2` and `0.3.1`) are compatibility
+shims depending on the *next* version of `rand_core`. When multiple crates
+depend on `rand_core`, they must *all* depend on the same version of `rand_core`
+*or* depend on a compatibility shim, e.g. `rand_core` versions `0.4.x` and
+`0.3.1` are compatible, `0.4.0` and `0.3.0` are *not*. Usually, running
+`cargo update` is enough to resolve any issues.
+
 The Rand lib is not yet stable, however we are careful to limit breaking changes
 and warn via deprecation wherever possible. Patch versions never introduce
 breaking changes. The following minor versions are supported:

rand_core/README.md

@@ -40,6 +40,18 @@ The `rand_core` crate provides:
 
 The traits and error types are also available via `rand`.
 
+## Versions
+
+Rand libs have inter-dependencies and make use of the
+[semver trick](https://github.com/dtolnay/semver-trick/) in order to make traits
+(particularly `RngCore`) compatible across crate versions. This means that a
+few crate releases (`rand_core` versions `0.2.2` and `0.3.1`) are compatibility
+shims depending on the *next* version of `rand_core`. When multiple crates
+depend on `rand_core`, they must *all* depend on the same version of `rand_core`
+*or* depend on a compatibility shim, e.g. `rand_core` versions `0.4.x` and
+`0.3.1` are compatible, `0.4.0` and `0.3.0` are *not*. Usually, running
+`cargo update` is enough to resolve any issues.
+
 ## Crate Features
 
 `rand_core` supports `no_std` and `alloc`-only configurations, as well as full