Add policy on yanked crates, add explanation for rand 0.7.1

Closes #893

Author: dhardy

CHANGELOG.md

@@ -16,6 +16,10 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 - Unit tests for value stability of distributions added (#888)
 
 ## [0.7.1] - 2019-09-13
+### Yanked
+This release was yanked since it depends on `rand_core::OsRng` added in 0.5.1,
+yet only specifies a dependency on version 0.5.0 (#890).
+
 ### Fixes
 - Fix `no_std` behaviour, appropriately enable c2-chacha's `std` feature (#844)
 - `alloc` feature in `no_std` is available since Rust 1.36 (#856)

README.md

@@ -67,6 +67,12 @@ A detailed [changelog](CHANGELOG.md) is available.
 When upgrading to the next minor series (especially 0.4 → 0.5), we recommend
 reading the [Upgrade Guide](https://rust-random.github.io/book/update.html).
 
+### Yanked versions
+
+Some versions of Rand crates have been yanked ("unreleased"). Where this occurs,
+the crate's CHANGELOG *should* be updated with a rationale, and a search on the
+issue tracker with the keyword `yank` *should* uncover the motivation.
+
 ### Rust version requirements
 
 Since version 0.7, Rand requires **Rustc version 1.32 or greater**.