feat: change README.md file to successfully install and use foundry

[eminmtas]

README.md file is not clear about installing foundry. When we use curl -L https://foundry.paradigm.xyz | bash to install foundry, it keep gives this warning:

Detected your preferred shell is bash and added foundryup to PATH. Run 'source /home/codespace/.bashrc' or start a new terminal session to use foundryup.
Then, simply run 'foundryup' to install Foundry.

Screeenshot:

We just have to change the source and use foundryup command and then we can continue.

I think README.md file should be clearer. Just adding some lines of explanation will make it more clearer.