message: Update the gcc compilers list with more version details. Remove now uneeded contribution note and cleanup some CUDA links.
diff: @@ -26,7 +26,7 @@ For Ginkgo core library:
  * _cmake 3.8+_
  * C++11 compliant compiler, one of:
  - * _gcc 5.5.0+_
  + * _gcc 5.3+, 6.3+, 7.3+, 8.1+_
  * _clang 3.9+_
  * _Apple LLVM 8.0+_ (__TODO__: verify)
  @@ -35,19 +35,13 @@ The Ginkgo CUDA module has the following __additional__ requirements:
  * _CUDA 9.0+_
  * Any host compiler restrictions your version of CUDA may impose also apply
  here. For the newest CUDA version, this information can be found in the
  - [CUDA installation guide for Linux](https://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html#system-requirements)
  - or [CUDA installation guide for Mac Os X](https://docs.nvidia.com/cuda/cuda-installation-guide-mac-os-x/index.html#system-requirements)
  + [CUDA installation guide for Linux](https://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html)
  + or [CUDA installation guide for Mac Os X](https://docs.nvidia.com/cuda/cuda-installation-guide-mac-os-x/index.html)
  In addition, if you want to contribute code to Ginkgo, you will also need the
  following:
  * _clang-format 5.0.1+_ (ships as part of _clang_)
  -* __NOTE:__ If you want to use _clang_ as your compiler and develop Ginkgo,
  - you'll currently need two versions of _clang_: _clang 4.0.0_ or older, as
  - this is this version supported by the CUDA 9.1 toolkit, and _clang 5.0.1_ or
  - newer, which will not be used for compilation, but only provide the
  - _clang-format_ utility.
  -
  ### Windows
  