Add Measurement and Scoring details to README.md (#467)

Document in more detail how scoring and workload measurement works:
- Add example timeline to explain which parts contribute to the score
- Add basic formulas on how the test steps are combined into a final score

Author: camillobruni

README.md

@@ -14,3 +14,25 @@ benchmark, actual users should benefit. In order to achieve this, it should:
 -   Be accessible to the public and useful to browser engineers. It should run in every modern
     browser by visiting a normal web page. It should run relatively quickly, while providing
     enough test coverage to be reflective of the real-world Web.
+
+# Score and Test Measurement
+
+Each test can contain several steps who contributed to the test duration. Note that the prepare step of a test is _unmeasured_ and thus does not contribute to the score. All following test
+steps are **measured** and summed up in the test step time.
+
+<img  src="./resources/measurement-timeline.svg" />
+
+Each step consists of the following phases:
+
+<dl>
+    <dt>Sync:</dt>
+    <dd>Time spent in synchronous JS execution.</dd>
+    <dt>Async:</dt>
+    <dd>Time spent in asynchronous JS like Promise or setTimeout tasks scheduled by the sync-step.</dd>
+</dl>
+
+## Score
+
+The final benchmark score is calculated based of the inverse of [geomean](https://en.wikipedia.org/wiki/Geometric_mean) of all tests. We average the score over multiple iterations with the [arithmetic mean](https://en.wikipedia.org/wiki/Arithmetic_mean). By using the geomean to combine the test durations we maintain the invariant that relative improvements are favoured equally amongst all tests, even though they can have vastly different durations.
+
+<img  src="./resources/score.svg" />