Update instructions to set fast-jar flag

[lordofthejars]

No description provided.

[jaikiran]

Failing with (of course unrelated to this PR):

2020-09-03T12:10:35.9036952Z [ERROR] Tests run: 7, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 117.644 s <<< FAILURE! - in io.quarkus.cli.CliTest
2020-09-03T12:10:35.9041895Z [ERROR] io.quarkus.cli.CliTest.testAddListRemove  Time elapsed: 0.908 s  <<< FAILURE!
2020-09-03T12:10:35.9046067Z org.opentest4j.AssertionFailedError: expected: <quarkus-resteasy> but was: <>
2020-09-03T12:10:35.9051889Z 	at org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:55)
2020-09-03T12:10:35.9057108Z 	at org.junit.jupiter.api.AssertionUtils.failNotEqual(AssertionUtils.java:62)
2020-09-03T12:10:35.9061798Z 	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:182)
2020-09-03T12:10:35.9065631Z 	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:177)
2020-09-03T12:10:35.9069315Z 	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:1124)
2020-09-03T12:10:35.9072874Z 	at io.quarkus.cli.CliTest.testAddListRemove(CliTest.java:90)

[jaikiran]

Given that this PR is about a change in the comment in a file and has been approved, I'll go ahead and merge this.

[ia3andy]

@gsmet we need to be really careful not to forget to update Codestarts templates too: #12166