Enhanced the README file by adding detailed maintenance policies for the
`meta-freescale` layer, including information about branch management
and release cycles.
Additionally, included specifics about the maintained branches such as
`master`, `scarthgap`, `nanbield`, and `kirkstone`. This update aims to
provide users with a clearer understanding of the project's maintenance
practices and available branches.
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>