Add tips and tricks

[b2renger]

• keep a big enough white border on the markers
• don't use big videos : shutter encore - use mp3 for audio
• barcodes can only be detected once : if you want the same contents to appear several times put it on several markers
• beware of the lightning conditions when in use
• have marker as mate as possible : don't use material that could reflect light (laser pinting is often too much glossy)
• ...