Describing migration steps in Readme from 9.2 onwards

[Symyon]

The last few releases had a few breaking changes. While the commit and discussion about it si linked, it is not trivial to apply the changes retrospectively for files that usually are not regenerated entirely like context, fetcher...

[fabien0102]

Very good point, my prefered workflow is:

• Delete every generated file
• Regenerate
• Commit what’s interesting (the utils functions, some type changes)
• Discard the rest

I will try to make a little guide, sorry for the inconvenience.