benchmarks: add README pointing at the shared matrix in SmartString

One shared 25-cell matrix covers both libraries since SmartArray's hot
paths end in SmartString output. The README says where the results,
scripts, and workflow live and how to re-run them.

Author: daveedis

benchmarks/README.md

@@ -0,0 +1,15 @@
+# Benchmarks
+
+SmartArray's speed benchmarks live in the SmartString repository: one shared
+matrix covers both libraries, since SmartArray's hot paths (field access,
+iteration) end in SmartString output.
+
+In [interactivetools-com/SmartString](https://github.com/interactivetools-com/SmartString):
+
+- **Results**: `.github/scripts/speed-results.md` - SmartArray tests have
+  `arr-` prefixed ids (property access, `get()` vs property, foreach iteration)
+- **Scripts**: `.github/scripts/speed-probe.php` (per-cell measurements) and
+  `speed-merge.php` (combines cells into the results grid)
+- **Workflow**: `.github/workflows/speed-matrix.yml` - dispatch it from the
+  Actions tab to re-run; 25 cells (PHP 8.1-8.5 x linux-x64/linux-arm/
+  windows-x64/macos-x64/macos-arm)