update README
Add heuristic head assignment rules

... for the sake of reproducibility.
Expand evaluation parameters to cover PTB, Alpino

- Default cutoff length of 40 words
- Ignore root labels TOP, VROOT; NOPARSE label
- Ignore punctuation with POS tags for each treebank
- As fallback, ignore punctuation based on word tokens
- Treat ADVP and PRT as equivalent, following COLLINS.prm