Default values for `readme` if not specified

[joshtriplett]

Many packages specify readme = "README.md". Could we automatically detect README.md,README, or README.txt, and use those as the values of readme if the files exist and the manifest key doesn't, just as we do for build? That would avoid the need to specify it in most common cases.

[joshtriplett]

We discussed this in the cargo meeting, and agreed we should do this.

Also, readme = false should disable this behavior, just as build = false disables the default of using build.rs if it exists.

[tverghis]

I'd like to take a stab at this!

[ehuss]

Thanks @tverghis! Let us know if you have any questions.