Bump version to 0.6.1 (#16)

Merge pull request #15 from ViliamVadocz/master

Bumped version

Explain and enforce the single-use exercise policy.

Add on_briefing() to version.py

Added an importer for RocketLeagueCustomStrikerTraining

Motivation:
https://www.youtube.com/watch?v=AMS01PdJ9Lw
https://docs.google.com/spreadsheets/d/1SUab3PF0dL-rjfuhKNUEx_4y9eOFA8dIAIZ-_7upcS0/edit#gid=0

Fix some directories missing __init__.py files