Document the timeline

Author: pavpanchekha

AGENTS.md

@@ -18,6 +18,19 @@
 - Additionally `--enable dump:rival` outputs all Rival commands it
   executes, which can be useful for debugging Rival &
   arbitrary-precision code.
+  
+# Timeline
+
+- The timeline is Herbie's observability tooling. Timeline information
+  goes into `timeline.json` and `timeline.html`.
+- A timeline is a list of phases, each of which is a hash table from
+  key to "table", where a table is a list of rows each of which is a
+  fixed-length array. Define new timeline types in
+  `src/utils/timeline.rkt` and add/change HTML generation in
+  `src/reports/timeline.rkt`.
+- Add to the timeline with `(timeline-push! 'type val1 val2 ...)`. The
+  `val`s must be JSON-compatible; this mostly means you have to
+  convert symbols to strings.
 
 # Documentation