Add comment to markdown on how to run EarlGrey tests.

EarlGrey tests are run differently than other iOS targets, so this
should be document.

Bug: 
Change-Id: I112108008a85f810b7e96fb0241b1c8277ec9ea9
Reviewed-on: https://chromium-review.googlesource.com/567918Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Mike Baxley <baxley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485951}

docs/ios/build_instructions.md

@@ -234,6 +234,18 @@ command line, you can use `iossim`. For example, to run a debug build of
 $ out/Debug-iphonesimulator/iossim out/Debug-iphonesimulator/Chromium.app
 ```
 
+### Running EarlGrey tests
+
+EarlGrey tests are run differently than other test targets, as there is an
+XCTest bundle that is injected into the target application. Therefore you must
+also pass in the test bundle:
+
+```shell
+$ out/Debug-iphonesimulator/iossim \
+    out/Debug-iphonesimulator/ios_chrome_ui_egtests.app \
+    out/Debug-iphonesimulator/ios_chrome_ui_egtests.app/PlugIns/ios_chrome_ui_egtests_module.xctest
+```
+
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