Add documentation for `mojo_benchmark`.

mojo/devtools/common/docs/mojo_benchmark.md

Index: mojo/devtools/common/docs/mojo_benchmark.md
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+# mojo_benchmark
+
+`mojo_benchmark` allows you to run performance tests for any Mojo application
+that participates in the [tracing
+ecosystem](https://github.com/domokit/mojo/blob/master/mojo/services/tracing/public/interfaces/tracing.mojom)
+with no app changes required.
+
+The script reads a list of benchmarks to run from a file, runs each benchmark in
+controlled caching conditions with tracing enabled and performs specified
+measurements on the collected trace data.
+
+## Defining benchmarks
+
+`mojo_benchmark` runs performance tests defined in a benchmark file. The
+benchmark file is a Python dictionary of the following format:
+
+```python
+benchmarks = [
+  {
+    'name': '<name of the benchmark>',
+    'app': '<url of the app to benchmark>',
+    'shell-args': [],
+    'duration': <duration in seconds>,
+
+    # List of measurements to make.
+    'measurements': [
+      '<measurement type>/<event category>/<event name>',
+    ],
+  },
+]
+```
+
+The following types of measurements are available:
+
+ - `time_until` - measures time until the first occurence of the specified event
+ - `avg_duration` - measures the average duration of all instances of the 
+    specified event
+
+## Caching
+
+The script runs each benchmark twice. The first run (**cold start**) clears the
+following caches before start:
+
+ - url_response_disk_cache
+
+The second run (**warm start**) runs immediately afterwards, without clearing
+any caches.
+
+## Time origin
+
+The underlying benchmark runner records the time origin just before issuing the
+connection call to the application being benchmarked. Results of `time_until`
+measurements are relative to this time.
+
+## Example
+
+For an app that records a trace event named "initialized" in category "my_app"
+once its initialization is complete, we can benchmark the initialization time of
+the app (from the moment someone tries to connect to it to the app completing
+its initialization) using the following benchmark file:
+
+```python
+benchmarks = [
+  {
+    'name': 'My app initialization',
+    'app': 'https://my_domain/my_app.mojo',
+    'duration': 10,
+    'measurements': [
+      'time_until/my_app/initialized',
+    ],
+  },
+]
+```