Add a readme file

[striezel]

This project has no README file yet.

However, a readme would be a good place to explain how optional-fun-lite works with std::optional / nonstd::optional. The lack of such an explanation or documentation may also be one reason why optional-fun-lite seems to be less popular than optional-lite (judging from the number of stars and forks).

[martinmoene]

Added initial much-left-to-be-desired Readme and forgot to mention this issue in its commit 24a6a68.