Need to document that `sd` does _not_ stream, loads entire input into memory

[Cornelius Roemer]

I was happy with sd until I noticed that it reads the entire input into memory. There's no streaming support at all.

This is mentioned in the following issues:

• #96
• #100
• #154

But I would expect this to be documented in the README, as it is a major caveat and limitation compared to standard sed.

[Orion (Ardi) Gonzalez]

Don't we mmap everything now? @nc7s

[Blair Noctis]

@dev-ardi:

Don't we mmap everything now? @nc7s

Stdin is still read in full into memory, it's then converted (wrapped?) to a mmap. See discussion in #287.