Clarify roles for guest editor recruitment

[noamross]

A few lines specifying what the Software Review Lead and Editor in Chief do in the case of bringing on a guest editor.

[noamross]

Also make sure EiC has appropriate permissions needed to onboard new and guest editors. (@adamhsparks, let's track missing permissions/steps here)

[maelle]

or, a way to request staff members to do so on their behalf. Less bureaucratic work for volunteers 😸