add linux installation docs; improve formatting of mac installation docs
add linux installation docs; improve formatting of mac installation docs

ARMmbed/yotta,theotherjimmy/yotta,stevenewey/yotta,eyeye/yotta,theotherjimmy/yotta,ntoll/yotta,ARMmbed/yotta,bridadan/yotta,ntoll/yotta,iriark01/yotta,BlackstoneEngineering/yotta,BlackstoneEngineering/yotta,bridadan/yotta,iriark01/yotta,stevenewey/yotta,autopulated/yotta,autopulated/yotta,eyeye/yotta