Add appropriate documentation

Author: Xaeroxe

src/doc/src/reference/unstable.md

@@ -84,6 +84,7 @@ Each new feature described below should explain how to use it.
     * [build-std](#build-std) --- Builds the standard library instead of using pre-built binaries.
     * [build-std-features](#build-std-features) --- Sets features to use with the standard library.
     * [binary-dep-depinfo](#binary-dep-depinfo) --- Causes the dep-info file to track binary dependencies.
+    * [checksum-freshness](#checksum-freshness) --- When passed, the decision as to whether a crate needs to be rebuilt is made using file checksums instead of the file mtime.
     * [panic-abort-tests](#panic-abort-tests) --- Allows running tests with the "abort" panic strategy.
     * [host-config](#host-config) --- Allows setting `[target]`-like configuration settings for host build targets.
     * [target-applies-to-host](#target-applies-to-host) --- Alters whether certain flags will be passed to host build targets.
@@ -512,6 +513,13 @@ the crate will be rebuilt). The primary use case is for building the compiler
 itself, which has implicit dependencies on the standard library that would
 otherwise be untracked for change-detection.
 
+## checksum-freshness
+* Tracking issue: [#14136](https://github.com/rust-lang/cargo/issues/14136)
+
+The `-Z checksum-freshness` flag will replace the use of file mtimes in cargo's
+fingerprints with a file checksum value. This is most useful on systems with a poor
+mtime implementation, or in CI/CD. The checksum algorithm can change without notice between cargo versions. Fingerprints are used by cargo to determine when a crate needs to be rebuilt.
+
 ## panic-abort-tests
 * Tracking Issue: [#67650](https://github.com/rust-lang/rust/issues/67650)
 * Original Pull Request: [#7460](https://github.com/rust-lang/cargo/pull/7460)