release_notes: update release notes for version 1.2.1

Signed-off-by: Héctor González <hector.gonzalez@digi.com>

fix digidotcom#53: check that 64-bit address is not unknown before ob…

…taining the

remote XBee device from the network

digidotcom#53

Signed-off-by: Ruben Moral <Ruben.Moral@digi.com>

XBee Java Library v1.1.1

XBee Java Library v1.1.0

XBee Java Library version 1.0.1

Updating the release notes.

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>