Add a guide for Python SPL operator support.

[wmarshall484]

No description provided.

[ddebrunner]

As a reference the API doc is here:

http://ibmstreams.github.io/streamsx.topology/doc/releases/1.6/pythondoc/streamsx.spl.spl.html

[jakrmpotic]

@ddebrunner some copy edits on the ref info:

Missing word (show how a ?) or typo (show a ?):
These examples how a SPL tuple with the schema and value:

Mostly use “a SPL”, sometimes use “an SPL”. Use one form consistently.

Missing the word “is”:
When a list returned, each value is converted to an SPL tuple and submitted to the output port, in order of the list starting with the first element (position 0).

Incomplete sentence:
If a tuple is returned then it is c

Typo, should be “derived”:
dervied from the value