Added lack-of-input-validation.md

[indeqs]

Added lack-of-input-validation as a response to issue 48

[kadenzipfel]

Will come back to do a proper review of this, but initial thoughts are:

• I think there's lots of ways to validate inputs on-chain which are commonly recommended, i.e. check that we aren't passing address(0) or a 0 amount when not expected
• Should external input validation be included in this repo?
  • I think if so then we should include it as a secondary consideration, i.e. "Here's how and why to validate inputs on-chain" and then supplementary, "Here's how and why to validate inputs off-chain"
• Maybe we can also talk about patterns like two-step ownership transfers which do a really good job for extra sensitive inputs

[indeqs]

After combing through some old docs, this issue was fixed in Solidity v0.5.0 as seen here and some more discussion on the same here

It was a learning journey!

I believe it is also worth a note to know the existence of this blog here as it goes into a little bit depth on the same