Add more information on pre-built ccached images to README. (#12994)

infra/experimental/chronos/README.md

@@ -1,5 +1,33 @@
 # Chronos: rebuilding OSS-Fuzz harnesses using cached builds
 
+## Pre-built images.
+
+Daily pre-built images are available at:
+
+- `us-central1-docker.pkg.dev/oss-fuzz/oss-fuzz-gen/<PROJECT>-ofg-cached-address`
+- `us-central1-docker.pkg.dev/oss-fuzz/oss-fuzz-gen/<PROJECT>-ofg-cached-coverage`
+
+They can be used as drop-in replacements for the usual `gcr.io/oss-fuzz/<PROJECT>` images.
+
+These images are generated in 2 ways:
+- (Preferred) [Generate](https://github.com/google/oss-fuzz/blob/master/infra/base-images/base-builder/bash_parser.py)
+  a replay build script that can be re-run alongside existing build artifacts,
+  leveraging existing build system mechanisms to avoid rebuilding (e.g. running
+  `make` twice should not actually rebuild everything). This is error-prone, so
+  we validate the script works by running it.
+- (Fallback, if the replay build script didn't work). We leverage
+  [ccache](https://ccache.dev/), to provide a compiler cache. This is often not
+  as fast as the replay build script, because some project builds spend
+  significant time doing non-compiler tasks (e.g. checking out submodules,
+  running configure scripts).
+
+Note: this mechanism does not work for every single OSS-Fuzz project today. The
+resulting image may either:
+- Not provide much performance improvement compared with a normal image, or
+- Not exist at all (if neither approach worked).
+
+TODO: Aggregate stats on expected performance improvements.
+
 ## Usage locally
 
 **Example 1: htslib**