doc: Update README
Add infrastructure for subscribing to data moving along edges

Not hooked up to commandline / UI yet, and can only shows the value
of the data for booleans.

Refs #8
Allow to configure max node and message capacity using preprocessing macros

For example with -DMICROFLO_NODE_LIMIT=10 -DMICROFLO_MESSAGE_LIMIT=10
SRAM usage is circa 350 bytes on AVR8.

Refs #16