Add oldSecrets field to add-member

[Powersource]

Based on #32 (merge that one first)

For ssbc/ssb-tribes2#106

• update readme

[Powersource]

@mixmix would it make sense to instead put this in the epoch inits? would save a lot of repetition, but would make initial replication of a group slower

(ps our meeting notes for reference ssbc/ssb-tribes2#76 (comment) )

[Powersource]

might have to add some special logic in tribes2 for this though which would mean more coding

[mixmix]

not a bad idea... but would rather not right now - the problem is that the group/init doesn't necessarily know about all past epochs.... oh, well it does actually, just not the parallel ones.

There might be a problem with how many keys you could add to a new init.
I kinda like the idea the more I think about it.

[Powersource]

Since this is coded already and it's not too bad I'm gonna put this in this "stuff we could maybe do if we have more time" issue ssbc/ssb-group-exclusion-spec#30 and just merge

[mixmix]

up to you if you want to mutate this or the group/init.

I don't know what the implications of replicating epochs in a funny order would be... note that we need to know all the people who started the epochs too to be able to replicate them D:

[Powersource]

v8