Have a look at the mainpage command.
Also, have a look this answer to another thread: How to include custom files in Doxygen. It states that there are three extensions which doxygen classes as additional documentation files: .dox, .txt and .doc. Files with these extensions do not appear in the file index but can be used to include additional information into your final documentation – very useful for documentation that is necessary but that is not really appropriate to include with your source code (for example, an FAQ)
So I would recommend having a mainpage.dox (or similarly named) file in your project directory to introduce you SDK. Note that inside this file you need to put one or more C/C++ style comment blocks.