[Updata READEME] rlberry has been updated, SofaGym only surpports 0.5.0

[0707yiliu]

The new rlberry==0.6.0 has been updated in the file structure, causing sofagym to error.
It should uses rlberry==0.5.

[samuelmyoussef]

Thank you for the notice, the readme is now updated #56.
I will look into supporting the newer versions of rlberry.