Update FilterSubscriber documentation.

[kherge]

Problem

In the 0.9.0 release, the ability to filter by exact paths was added. Unfortunately, the documentation¹ does not reflect the availability of this new feature.

Resolution

I would like to see information about using exact path filtering with the FilterSubscriber in the README.md file.

References

1. https://github.com/Sqon/sqon/tree/0.9.0#filtersubscriber

[kherge]

Fixed by d7c235b.