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

[corneliusroemer]

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:

• Massive memory usage for large file #96
• Follow STDIN input #100
• Bug: Memory allocation failure #154

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

[oriongonza]

Don't we mmap everything now? @nc7s

[nc7s]

@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.