Document changes for inmemory backend with Janusgraph 0.5.0 upgrade

[FieteO]

---

Update changelog to contain update instructions for changes to in-memory backend introduced with 0.5.0.

Fixes #4532

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[FlorianHockmann]

This looks good to me. Thanks for the contribution!

[porunov]

LGTM. Thank you @FieteO !

[janusgraph-automations]

💚 All backports created successfully

Status	Branch	Result
✅	v1.0

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