pubsub library not thread safe.

[jonasbjurel]

Super good MQTT library - kudos!
It appears that the library is not thread safe.
Introducing lock protecting buffer may be difficult while supporting all hardware .
I propose to solve this by documentation:
"The pubsub library is not thread safe, in case several threads use the library following methods need to be protected
from concurrent use:

• connect()
• disconnect()
• publish()
• loop()"