Link to metrics docs from the main docs/README.md

R=mpearson@chromium.org

Bug: none
Change-Id: I0eb5768c4e24b585f19ceb352b59e3728a19ab5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415217
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807943}

docs/README.md

@@ -348,6 +348,11 @@ used when committed.
 *   [Memory Benchmarks](memory-infra/memory_benchmarks.md)
 *   [Heap Profiling](memory-infra/heap_profiler.md)
 
+### Metrics
+*   [Histograms](/tools/metrics/histograms/README.md)
+*   [User Actions](/tools/metrics/actions/README.md)
+*   [Code review guidelines](/tools/metrics/histograms/review_guidelines.md)
+
 ### Misc
 *   [Useful URLs](useful_urls.md) - A collection of links to various tools and
     dashboards