meintion need for explicit -i for wall profile in README.md

[razmser]

Issue: https://youtrack.jetbrains.com/issue/IDEA-320011
Mentioning this will probably save somebody's time.

[apangin]

Reading the linked IDEA issue I see it doesn't come from async-profiler by any means.
So there is nothing to do on async-profiler end, isn't it?

[razmser]

Yes. But IDEA issue isn't yet fixed. Note about it will be helpful and doesn't take much space.

[apangin]

Since #740, the proper option for configuring wall clock profiling interval is --wall.
I would not like to add misleading instructions just to warn somebody of a third party bug.
Thank you for understanding.