readme: update for next dev release

Bump version described in README to a development version of the next release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/boot/zephyr/VERSION b/boot/zephyr/VERSION
index f646e74..428a355 100644
--- a/boot/zephyr/VERSION
+++ b/boot/zephyr/VERSION
@@ -1,5 +1,5 @@
 VERSION_MAJOR = 2
-VERSION_MINOR = 0
+VERSION_MINOR = 1
 PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION =
+EXTRAVERSION = dev