Add example of GPIO input mode

[YuaXan]

I not found example about gpio input mode long time, so I written the example to share everyone.

[YuaXan]

Not guys come pull requests? @TheZoq2 @therealprof

[therealprof]

@YuaXan Thanks for the PR! I think we could certainly use more examples. There're some things which need improving though to make it an educational resource:

• More code comments
• Spellcheck
• Addition to the Cargo.toml

Also the manual re-implementation of the OutputPin (and other) traits is a bit unusual. Why not use type aliases instead to define your custom RedLedPin and GreenLedPin?

[YuaXan]

@therealprof Thanks for your tell me about my mistakes. I fixed these mistakes.

[burrbull]

bors r+

[bors]

Build succeeded:

• ci (stm32f100, stable)
• ci (stm32f101, stable)
• ci (stm32f103, stable)
• ci (stm32f105, stable)
• ci (stm32f107, stable)
• Rustfmt

[YuaXan]

@burrbull Thanks for merged my PR, This was my first contribution to opensource. I wish I will keep going. :D

[TheZoq2]

@YuaXan nice job on your first contribution! Sorry for ignoring this (and a bunch of other PRs) for so long, I hope that didn't put you off too much!

And thanks @burrbull for taking care of all the PRs which i have ignored ❤️