minor #2545 Mention the Rector PHP set in the UPGRADE guide (javiereguiluz)
minor #2545 Mention the Rector PHP set in the UPGRADE guide (javiereguiluz)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Mention the Rector PHP set in the UPGRADE guide

As mentioned by our friend Tomáš (@TomasVotruba) in [this comment](https://github.com/EasyCorp/EasyAdminBundle/pull/2494#issuecomment-448320019).

Commits
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26f93131 Mention the Rector PHP set in the UPGRADE guide

yceruto/EasyAdminBundle,yceruto/EasyAdminBundle,yceruto/EasyAdminBundle,javiereguiluz/EasyAdminBundle,OskarStark/EasyAdminBundle,javiereguiluz/EasyAdminBundle,OskarStark/EasyAdminBundle,OskarStark/EasyAdminBundle