Initial changes to support substreams

[cristian-recoseanu]

• Create substream_constraints.json
• Update constraint_set.json to include the new substreams property
• Update Examples.md to add MUX example using substreams

[cristian-recoseanu]

Looks good. Suggest review use of capitalisation ("Mux")

Now addressed in commit 96e4c524089583bead764cf8598bfbca2097dbce