Mention missing breaking change in ThemisPP (#675)
Mention missing breaking change in ThemisPP (#675)

We mention this PR as a minor refactoring in Themis Core, but in fact it
includes a breaking change in ThemisPP as well. This change affects all
users of Secure Session and breaks compilation. Let's mention it in the
changelog to simplify migration.
cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis