Update README.md

Decapitalised secondary headings and linked the *Credits* section
Merge pull request #1867 from felixphew/patch-2

Update README.md
Merge remote-tracking branch 'arduino/master' into ide-1.5.x
Merge branch 'ide-1.5.x' into ide-1.5.x-avr-toolchain
Merge branch 'ide-1.5.x-avr-toolchain' into ide-1.5.x-avr-toolchain-gcc-4.8.1