[vigir_ltl_synthesizer] Update synthesis_byproducts README

Author: spmaniato

vigir_ltl_synthesizer/synthesis_byproducts/README.md

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+# Synthesis Byroducts
+
+The `slugs` synthesizer needs to generate a few files each time it synthesizes.
+File formats include `.json`, `.slugsin`, and `.structuredslugs`.
+These are saved in a folder named after the specification and can be useful for debugging the LTL specifications and resulting automata.
+
+The repository's `.gitignore` is set up to ignore everything in the `synthesis_byproducts` folder except for this `README` file.