Explain the choice of editor (core.editor)

[robmoss]

@nefeltellioglu identified that we should explain what this settings means, and how it affects your workflow. In particular:

• Using an IDE or graphical editor may make it difficult for Git to determine when the commit message has been finalised/discarded;
• Console-based editors such as vim or nano can be difficult for new users (e.g., entering insert mode, saving, and quitting vim);
• This editor is only used for writing commit messages, it is completely independent of your choice of editor for the files that you are tracking with Git.

[robmoss]

This example of setting the editor to sublime, and the subsequent comments, provide some ideas about the potential challenges and pitfalls.