courses/rust/projects/project-1: Add note about clap 3.x

[myrfy001]

What problem does this PR solve?

• Issue number: close #332
• As of clap 3.x has ben released, we need to update the guide.

What is changed and how it works?

• We add a note at the top of the guide, letting the readers to find out the difference between clap 2.x and 3.x by themselves. By this way, we can make the reader to do more exercise on reading docs and make them get used to the quickly changing rust ecosystem.
• The readers can decide to still use the 2.x or upgrade to the 3.x by themselves, both way is ok.

Check List

Tests

• No code

[yanguwan]

"and after knowing the difference, you can choose to skip some of the steps in this guide."

[yanguwan]

"and after knowing the difference, you can choose to skip some of the steps in this guide." Can we point out what steps should be skipped once clap 3.x is used.

[myrfy001]

In my opinion, reading the document and change log of other opensource projects is also a challenge. I think we shouldn't tell how to do it directly.