Create new `deprecation` label

[juangirini]

Despite the efforts for reducing the amount of labels I believe we need to create a new one that indicates a special kind of issue, which is a deprecation issue.
This is why I believe we need a new label: we are creating a new deprecation process, in which created issues need to follow a special flow. As you can see in that linked issue, this flow is different because deprecating a feature requires some async events, such as updating the documentation or actually removing the feature.
Currently we are relying in keywords to filter such issues, but obviously this isn't optimal.

[juangirini]

@kianenigma @the-right-joyce @ggwpez

[juangirini]

Might be fixed by #29

[the-right-joyce]

@juangirini I've already included the deprecation label on the list (see I12-deprecation)

[kianenigma]

Yeah it is already there, I think this should be closed.

[juangirini]

Closing this to have a unified thread on paritytech/substrate#29