Add examples to the partitionkey extension

[duglin]

See: #218

@jroper @clemensv - could one of you consider taking care of this one?

[duglin]

Clearly not required for v1.0 but is this something that someone would like to try to get added during our "spec review" period? Extra docs are always a good thing :-)

@clemens @bluemonk3y ?

[clemens]

This is not the Clemens you're looking for. ;)

[duglin]

LOL sorry - @clemens

@clemensv see above ^^^

[fnobilia]

Is it still needed? I'm happy to give it a try!

[MagicJohnJang]

How to use partition key on v2 ?

[MagicJohnJang]

NM, I got it here :
https://github.com/cloudevents/sdk-csharp/blob/1b9c7cc8c790eb89dd825dd8b50794b49e6a4018/test/CloudNative.CloudEvents.UnitTests/Extensions/PartitioningTest.cs#L51

[duglin]

This issue is running the risk of being closed soon. If you'd like to volunteer to drive it please speak up soon!

[duglin]

@xibz will take a look at this one...will revisit again soon

[jskeet]

Suggest we close unless we get a volunteer to do this imminently.

[duglin]

@xibz did you still want to find time to work on this one?