[test] Add more documentation about test-specifications

NOTRY=true
NOPRESUBMIT=true
NOTREECHECKS=true

Bug: chromium:788104
Change-Id: I1918d4ba69b2c00151ac51ee9426d980e24ab871
Reviewed-on: https://chromium-review.googlesource.com/808389
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49863}

infra/testing/README.md

 # Src-side test specifications
 
-The infra/testing folder in V8 contains test specifications, consumed and
-executed by the continuous infrastructure. Every master has an optional file
-named `<mastername>.pyl`. E.g. `tryserver.v8.pyl`.
+Src-side test specifications enable developers to quickly add tests running on
+specific bots on V8's continuous infrastructure (CI) or tryserver. Features to
+be tested must live behind runtime flags, which are mapped to named testing
+variants specified [here](https://chromium.googlesource.com/v8/v8/+/master/tools/testrunner/local/variants.py).
+Changes to src-side test specifications go through CQ like any other CL and
+require tests added for specific trybots to pass.
+
+The test specifications are defined in a V8-side folder called infra/testing.
+Every master has an optional file named `<mastername>.pyl`. E.g.
+`tryserver.v8.pyl`.
 
 The structure of each file is:
 ```
@@ -21,10 +28,10 @@ The structure of each file is:
 The `<buildername>` is a string name of the builder to execute the tests.
 `<test-spec name>` is a label defining a test specification matching the
 [infra-side](https://chromium.googlesource.com/chromium/tools/build/+/master/scripts/slave/recipe_modules/v8/testing.py#58).
-The `<variant name>` is a testing variant as specified in
-`v8/tools/testrunner/local/variants.py`. `<number of shards>` is optional
-(default 1), but can be provided to increase the swarming shards for
-long-running tests.
+The `<variant name>` is a testing variant specified
+[here](https://chromium.googlesource.com/v8/v8/+/master/tools/testrunner/local/variants.py).
+`<number of shards>` is optional (default 1), but can be provided to increase
+the swarming shards for long-running tests.
 
 Example:
 ```
@@ -47,4 +54,17 @@ tryserver.v8:
 client.v8:
   V8 Linux64
   V8 Linux64 - debug
-```
\ No newline at end of file
+```
+
+Please only add tests that are expected to pass, or skip failing tests via
+status file for the selected testing variants only. If you want to add FYI tests
+(i.e. not closing the tree and not blocking CQ) you can do so for the following
+set of bots:
+
+```
+tryserver.v8:
+  v8_linux64_fyi_rel_ng_triggered
+client.v8:
+  V8 Linux64 - fyi
+  V8 Linux64 - debug - fyi
+```