Update README.markdown

Source map info
Update README.markdown
Chrome Profiling is an eye opener
- fixed non optimized try catch in doChunk (big diff)
MarkerParentModel
- no chunking until updates
- never set existingPieces to undefined as it causes (thread / mutex)
  issues, think of existingPieces as the lock object
Logger
- logLevels default to error at startup
- doLog is now true with error only logging by default
- if logLevel is down to info or log, expect performance hits
issue #904
issue #798 as an example has been created and doChunk fixes help it as
well
Merge remote-tracking branch 'origin/master' into issue_904_markers_race_perf