Document Echidna current limitations

[montyly]

For example, Echidna does not work with immutable (#505), but we cannot expect the users to go through our list of issues to know that.

We should list the features that are not supported in the readme or wiki

[ggrieco-tob]

This feature will be supported "soon" when the code for that fix is merged.

[montyly]

I think there are a couple of other limitations; the one I can at least think of is for codebase with libraries that need to be manually deployed (#514).

Having a clear list of limitations that we can update over time will help the usability imho

[montyly]

#601 is another example of limitation that should be easily visible to the users

[montyly]

EOAs don't work, unless they are specified in sender: #648

[hacker-DOM]

External libraries don't always work #649

[ggrieco-tob]

A draft to document all these is here: #655

[ggrieco-tob]

This should be up to date here: https://github.com/crytic/echidna#limitations-and-known-issues