Added summary of key features and some formatting

I thought it might be handy to have a list of key features on the 
github page so that if someone runs into it looking for a compression
library they can know right away if it's what they're looking for.
Merge pull request #1 from jvranish/patch-1

Added summary of key features and some formatting
Reformatting/rephrasing feature summary
Include stddef.h for size_t.
Consistentcy: log to stderr for both enc and dec
Tighten bounds for lookahead and window size.

Since they're only stored as `uint16_t`s, disallow overly large sizes.
Change logging to match sizeof (%zu / %zu); print accumulator in hex
When index or count are > 8 bits long, ensure suspend/resume is correct.

Previously, suspending between the two bytes of a >8-bit index or count
could lead to accumulation of an incorrect value. This commit breaks
each of the index and count states into X_{msb,lsb}, where _msb reads
the >8th bits and stores them in the accumulator, and _lsb reads the
lower 8 bits (or all of them, if the total value is < 8). This adds
another point where the state machine can suspend and resume without any
loss of information, if necessary.
API change: change uint16_t to size_t for user-controlled buffer sizes.

This eliminates the possibility of infinite loops caused by counters
overflowing and preventing the output buffer full or input exhausted
conditions from being reached.
Add comments for decoder_poll states
Bump version to 0.2.0 due to API change.