e2e-tests: Add support for log level configuration

Document how to set log levels for sbt commands.

Add a logback.xml configuration file that sets Gatling's own log level.
Mention that file in the documentation, too.

Change-Id: I9b7726d776dd153f3717cb9bf206aa99eb20b6f1

Documentation/dev-e2e-tests.txt

diff --git a/Documentation/dev-e2e-tests.txt b/Documentation/dev-e2e-tests.txt
index afa3d68..77ef019 100644
--- a/Documentation/dev-e2e-tests.txt
+++ b/Documentation/dev-e2e-tests.txt
@@ -43,6 +43,14 @@
 [warn] Credentials file ~/.sbt/sonatype_credentials does not exist
 ----
 
+Every `sbt` command can include an optional log level
+link:https://www.scala-sbt.org/1.x/docs/Howto-Logging.html#Change+the+logging+level+globally[argument].
+Below, `[info]` logs are no longer shown:
+
+----
+sbt --warn compile
+----
+
 === How to build using Docker
 
 ----
@@ -106,6 +114,9 @@
 sbt "gatling:lastReport"
 ----
 
+The `src/test/resources/logback.xml` file
+link:http://logback.qos.ch/manual/configuration.html[configures] Gatling's logging level.
+
 === How to run using Docker
 
 ----