Describe process by which a contributor can become a core developer (#2959)

* Describe process by which a contributor can become a core developer

* Update GOVERNANCE.md

Co-Authored-By: hadley <h.wickham@gmail.com>

* Update GOVERNANCE.md

Co-Authored-By: hadley <h.wickham@gmail.com>

* Tweaking language

Author: hadley

GOVERNANCE.md

@@ -11,7 +11,9 @@ This project is led by a benevolent dictator and managed by a core team of devel
 The job of the benevolent dictator is to set the strategic objectives of the project and communicate these clearly to the community, ensuring that the project survives in the long term. 
 
 ## [Core Developers](https://github.com/orgs/tidyverse/teams/ggplot2)
-Core developers are contributors who have made several valuable contributions to the project and are now relied upon to both write code directly to the repository and screen the contributions of others. Core developers are empowered to merge pull requests after careful review. Core developers have no authority over the overall direction of the project, however it is their job to help develop or elicit appropriate contributions to the project. As a matter of policy, core developers, even if no longer active, are listed as [package authors](https://ggplot2.tidyverse.org/authors.html).
+Core developers are contributors who have made several valuable contributions to the project and are now relied upon to write code directly to the repository and screen the contributions of others. Core developers are empowered to merge pull requests after careful review. Core developers have no authority over the overall direction of the project, however it is their job to help develop or elicit appropriate contributions to the project. As a matter of policy, core developers, even if no longer active, are listed (chronologically) in [package authors](https://ggplot2.tidyverse.org/authors.html).
+
+An invitation to join the core team will be extended to contributors who have made a major contributions as a contributor. For example, they have added a major new feature or have a consistent pattern of smaller contributions over an extended period of time. Any existing core developer can propose a contributor be invited to the core team, pending confirmation by existing authors.
 
 ## Contributors
 Contributors are community members who make valuable contributions, such as those outlined in the list below, but generally do not have the authority to make direct changes to the project code. Instead, contributors can suggest changes to project code through the pull request process outlined in the project's [CONTRIBUTING](https://github.com/tidyverse/ggplot2/blob/master/CONTRIBUTING.md) document.