Mention IsolateNameServer on isolates.md

[mmmeeedddsss]

I had a use case where I needed to communicate between isolates. The isolate that needs to send a message was not being created by me but created by a library(flutter_local_notification's notification action)

During my search for a solution to create a communication channel in such a case, I found the excerpt that is just above my addition very confusing since it makes me believe that there is no solution to my problem. (Since I cannot interfere with the Isolate.spawn call directly, I thought I cannot obtain a communication channel between the isolates.)

This is why I'm doing that contribution to the doc. In my opinion, it is essential to mention the IsolateNameServer class inside isolates.md file, and state that it can allow dynamical registration of ports even though you cannot control the creation of the isolate.

I am very open for any suggestions and other changes to make that mention more blended to the rest of the document.

Thank you in advance!

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