Mark repository as archived?

[pierre-haessig]

I was about to post an issue here about the discrepency between the doc of some https://esa.github.io/pagmo2/docs/cpp/algorithms/pso.html and the actual code in that repo... before I realized I was not on the latest repo ;-)

Perhaps marking this repository as archived would help signaling things a bit stronger (I mean stronger in the sense that the README is already non ambiguous, but I had jumped straight in the source tree...) for occasional visitors like I.

[darioizzo]

Good suggestion! Indeed not too clear that this is old code.