Add instruction for renaming .env.example on the client subdirectory

[rcamach7]

When cloning the repo, on the frontend the app uses an environment variables to define API addresses. To get it working, you must rename .env.example to .env so it reads it correctly.

Under "How To Run", this instruction exists for the backend, but not for the frontend. Include this additional step on the .readme file

[rafaover]

Assign! me. Tks.

[lugenx]

hi @rafaover, thanks for helping. I can assign this issue to you after your other pr is merged.