Add retrocompatibility guidelines to documentation

[xroynard]

Add these informations to the Contributing page:

• API Retrocompatibility #97
• Disk Format Retrocompatiblity #14
• use decorators available in plaid.utils.deprecation
• use plaid version saved in Dataset and ProblemDefinition infos, thanks to 🎉(deprecation) add plaid version in infos of Dataset and ProblemDefinition #206