Incluir Guia de Contribuição para a comunidade

[erlimar]

Incluir o guia com informações e critérios para a comunidade contribuir com o projeto.

Subtarefa de #8!

Critério de aceitação

• Deve existir o aquivo docs/CONTRIBUTING.md;
• O arquivo docs/CONTRIBUTING.md deve estar referenciado em README.md;

Links úteis

• https://help.github.com/articles/setting-guidelines-for-repository-contributors
• https://github.com/dotnet/cli/blob/master/CONTRIBUTING.md
• https://github.com/dotnet/cli/blob/master/Documentation/project-docs/developer-guide.md
• https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/contributing.md
• https://github.com/dotnet/coreclr/blob/master/Documentation/project-docs/contributing.md
• https://github.com/dotnet/coreclr/blob/master/Documentation/project-docs/contributing-workflow.md
• https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/project-guidelines.md
• https://cakebuild.net/docs/contributing/contribution-guidelines
• https://github.com/Microsoft/visualfsharp/blob/master/CONTRIBUTING.md