Add a section about production build ui bug in docs

[edouardmenayde]

I struggled for hours with this and by chance found the issue talking about it but the next person might not have the same luck I did thus I think documenting the possible bug is important.

[fzaninotto]

We're using @material-ui/core@^1.4.0, which means that any version older than 1.4.0 will work. Which version were you using?

[djhi]

Rather than linking to the issue, it would probably be better to add the instructions directly here

[edouardmenayde]

@fzaninotto I was using an older version of react-admin and an older version of material ui core. I don't understand what was happening but nontheless I think it's important for it to appear in the docs.
@djhi I'm not sure because the issue will update itself easier than the docs. + The instruction in the wiki are general and good enough for anyone to fix its problem.

[fzaninotto]

You should probably rephrase this sentence as follows:

You have probably specified a version requirement for @material-ui/core that is incompatible with the one required by react-admin. As a consequence, npm bundled two copies of material-ui in your application, and material-ui doesn't work in that case.

Please align your version requirement with the one of the ra-ui-materialui package.

[fzaninotto]

Thanks!