Update README.md to Include License Information

[ryanbeaz]

Please determine the appropriate License for the source code and documentation of the SPDX_Eclipse_Plugin and update the README.md file to denote these. Additionally, please add the following license files to the branch: SourceLicense (which outlines the license for the source code) and DocumentationLicense (which outlines the documentation license).

[ryanbeaz]

After looking at differences between multiple open source licenses, I choose the following licenses for the SPDX_Eclipse_Plugin:

Source: Apache 2.0
Documentation: Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)

I also created the SourceLicense and DocumentationLicense files and included them in project. I also updated the README.md file to include other information relative to these licenses.