Updates to cross program invocation

[dadepo]

This PR adds the following:

• Updates the README to explain how the Cargo features is used to ensure there is only one entrypoint! when a Solana program depends on another Solana program
• Simplifies how the Cargo feature is used

[ngundotra]

Readme changes seem quite similar to #13, can you move your README changes to #13 only please?

Also, what do you mean by lever and hand crate? Otherwise, your additional docs look fine

[ngundotra]

Please add a comment to this PR explaining your changes

[dadepo]

Readme changes seem quite similar to #13, can you move your README changes to #13 only please?

Done

Also, what do you mean by lever and hand crate? Otherwise, your additional docs look fine

I was referring to this(lever) and this(hand). I referred to them by the rust convention of crates.

[buffalojoec]

If you moved this over from #10 can we close #10 and merge #13?

[dadepo]

If you moved this over from #10 can we close #10 and merge #13?

sure!

Moved the changes here to #13. Closing as suggested