Readme: bump version to 1.9.0-dev

Bump version to development version of future release.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/README.md b/README.md
index 5274b6c..f639342 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@
 [travis]: https://travis-ci.org/mcu-tools/mcuboot
 [license]: https://github.com/mcu-tools/mcuboot/blob/main/LICENSE
 
-This is MCUboot version 1.8.0
+This is MCUboot version 1.9.0-dev
 
 MCUboot is a secure bootloader for 32-bits microcontrollers. It defines a
 common infrastructure for the bootloader and the system flash layout on