Write documentation on how to set up mbeddr.cpp development environment

[eugenschindler]

Very simple mbeddr-like way:

• Installation of mbeddr prerequisites (specific version of toolchain etc. --> link and resuse here the specific versions)
• Clone
• Run gradlew to download all relevent dependencies (mbeddr.platform, mps extensions, mbeddr)
• Open project in MPS and set mbeddr.cpp.home path variable and let mps reopen the project so that proper project libraries from dependencies/ are loaded
  Caveat: do NOT use your own pre-packaged MPS (with extra stuff in its plugins directory), but use a VANILLA MPS.