Mention feature flags in README #657

Job	Run time
clippy-pedantic (tzdb)	9s
lint (1.81)	21s
clippy-pedantic (tzdb_data)	12s
lint (1.81, -Zminimal-versions)	22s
lint (stable)	10s
audit	18s
lint (stable, -Zminimal-versions)	16s
DevSkim	33s
cross-miri (aarch64-unknown-linux-gnu)	39s
lint (nightly)	18s
doc	16s
feature-powerset	15s
cross-miri (i586-unknown-linux-gnu)	39s
lint (nightly, -Zminimal-versions)	19s
cross-miri (powerpc-unknown-linux-gnu)	36s
fmt	18s
Miri	50s
cross-miri (powerpc64-unknown-linux-gnu)	35s
test	36s
cross-miri (powerpc64le-unknown-linux-gnu)	47s
typos	6s
	8m 35s

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