Minor changes to Communication Examples

Added a note to the included Processing sketches about replacing
println(Serial.list()) with Serial.printArray() if using Processing 2.1
or later
Update readme.txt
Update and rename readme.txt to README.mkdn
Rename README.mkdn to README.md
Updated Listfiles SD example

Changed SS pin to 4 for consistency with other examples
Merge pull request #1836 from KonradIT/master

Use markdown lang in the readme.
Merge remote-tracking branch 'arduino/master' into ide-1.5.x