Bump to 1.10.0-dev for development of next release

Bump the readme's version number to the next dev release.
Signed-off-by: David Brown <david.brown@linaro.org>

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 1.9.0
+This is MCUboot version 1.10.0-dev
 
 MCUboot is a secure bootloader for 32-bits microcontrollers. It defines a
 common infrastructure for the bootloader and the system flash layout on