Document approaches to working with macros

[harrah]

Because macro definitions need to be compiled separately from calls to the macro, it is beneficial to document how to configure projects to easily handle this. There are several aspects that different users will want to configure differently, such as:

• whether the macro is published
• shared interfaces between the macro implementation and usage
• what is included in scaladoc
• merging the macro and main project in the published jar

[harrah]

IMPORTANT This project has moved to https://github.com/sbt/sbt. This issue still exists to avoid dead links, but GitHub limitations prevent the original issue from being correctly preserved here. Please see sbt/sbt#573 for the full issue and to comment.