Expand and Verify Contributing Guide

[roseg31]

The instructions in the contributing guide need to be verified for clarity and completeness - they should be tested and any additional information needed should be added.

Additionally, these changes should be made:

• Fix formatting: list formatting should be either bullets or numbers but not both
• 'To Contribute' section should be cleaned up and more detailed, the multiple parentheses make it confusing. This section should probably reference GitHub documentation on how to execute pushing/pulling/pull requests.
• There needs to be a reference to style guidelines as described in Add Style Guide to Contributing Guildines #77
• From Ambiguous instructions on contributing to the project in Contributing.MD #34: The instructions should be clear about which repository should be used to create a branch i.e. the main repository or the fork (Should probably be a branch from the contributor's fork)
• From Ambiguous instructions on contributing to the project in Contributing.MD #34: In step three the phrase pop-up box should be removed.

[burnettm]

Also, please add advice about how to "onboard" to the code repository (ie, what to look at first, how to get a big picture).