Add `hoa\devtools` to suggest package in composer.json

As of #29 mentioned, the usage of flex entity will break some static analyzer. It might be a good idea to add hoa\devtools to suggest package.

[Hywan]

That's a good idea.

Thanks! I think we can add it to hoaproject/Central#75. Should we integrate it as part of our roadmap @vonglasow & @shulard?

[shulard]

Seems a good idea to reference the hoa/devtools package in suggest but we need to find the right sentence to use to describe the suggest meaning 😄.

[Hywan]

To polish the integration of Hoa inside your IDE, consider installing hoa/devtools.

I don't know if it describes well the situation though.

[shulard]

When installing hoa/devtools, does it fix the flex entity "problem" with static analyzer ?

@shulard Yes. At least in phpStorm.

[shulard]

Ok so the sentence LGTM, I've created a PR : #31.