Add a note about plugin specific settings #38

Job	Run time
lint	6s
docs / test	46s
base-images	18m 22s
app-images (nightly, pulp-minimal)	4m 35s
app-images (nightly, pulp)	4m 53s
app-images (nightly, galaxy-minimal)	8m 48s
app-images (nightly, galaxy)	9m 58s
app-images (stable, pulp-minimal)	9m 18s
app-images (stable, pulp)	23m 37s
app-images (stable, galaxy-minimal)	7m 24s
app-images (stable, galaxy)	9m 2s
	1h 36m 49s

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