Add brief style guidelines.

[pelesh]

This should probably be merged to develop, but for opportunistic convenience, I suggest we merge it here. Since #1 will likely prompt discussion on new style guidelines, let's direct this PR to composer-dev branch.

[stonecoldhughes]

Looks good. I would like to note that whispers of C++20 have reached even into the Contribution.md file. Perhaps something to revisit in the future.

[alexander-novo]

I think it would also be a good idea to set a (soft) column limit of 120 characters.

[reid-g]

Some details that I think should be added for clarification.

Initializing classes with the :. The member initializer list can add some weird issues on readability when there are a large number of them. This document should address this.

Structures localized as parameter inputs. When there are a lot of parameters it becomes a lot more manageable to use structures to handle them. Perhaps add some details on this?

These points come from details in the DistributedGenerator.hpp file.

Also with regards to file names and classes. Perhaps state that files with only one class names must match the name of their file (or something similar).

Otherwise looks good to me.

[alexander-novo]

I concur with @reid-g's points

[pelesh]

Structures localized as parameter inputs. When there are a lot of parameters it becomes a lot more manageable to use structures to handle them. Perhaps add some details on this?

This is important issue but somewhat out of scope of this PR. @reid-g would you mind creating a separate GitHub issue with details and examples?