add instructions for how to compile binary to README

[connerk]

The README lacks any instruction on how to properly assemble VBA-Import-Export.xlam from source.

[mattpalermo]

I have written a little bit about it in ./doc/release-instructions. When you build and open the VBA-Import-Export.xlsm file you can then save it as a .xlam file. The difference between the files is very little except for an "Addin" property set to true and the file name.