Replying to potential professor about my position I sent an email to  Professor A sometimes ago and he has recently showed his interest in my work and replied my email. He is asking about what is my research plan for my future. At the moment I am a postdoc in another country and I am looking for a postdoc position with respect to my research goal. In this respect, I have recently  applied for a postdoc position such that the result of accepted candidate will be announced soon (probably next week). My question is: Is it necessary to write about this details ( my recent application for postdoc position) in my replying email to Professor A?
You could easily reply now and say you are looking for a position and have made some applications. Just start the conversation. It may be that by the time you get a reply from him you will know more about your other prospects. 

Perhaps the first mail need not be very detailed, but you could ask about the possibility of working with him if you haven't done that already. 

It might be valuable to keep in touch even if you wind up with the other position. Nothing is forever and a circle of collaborators is valuable. 

And you might get a reply around the time that you hear about the other position and can make some decisions or follow up more seriously.