Update the developer notes

[venpopov]

They are already a bit behind, but after #152 is merged, definitely need to update. I should also add a note at the begining about the bmm version number associated with the developer notes, in case we change something again at some point and forget to update them - then at least it will be clear up to what version they are accurate.

[venpopov]

• Add conventions on coding style, variable naming etc to developer notes (as per conventions on variables, arguments, functions, etc #163 )