Switch to a branch-based release model (#382)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Pyribs roughly follows a trunk-based development model. Previously, for
releases, we only released from the trunk (master branch). However, this
does not work well if we need to perform a patch release while trunk
already has several features implemented for the next release. This PR
shifts to the [branch-based release
model](https://trunkbaseddevelopment.com/branch-for-release/) where we
create release branches for minor versions. If we need to release
patches, we can cherry pick commits from trunk into the release branch
and then push the patch release. We will use this to properly release
0.6.2, as 0.6.1 was released with features for 0.7.0 (namely, OMG-MEGA
and OG-MAP-Elites).

## TODO

<!-- Notable points that this PR has either accomplished or will
accomplish. -->

- [x] Update instructions in CONTRIBUTING.md
- [x] Update HISTORY.md

## Questions

<!-- Any concerns or points of confusion? -->

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [x] I have added a one-line description of my change to the changelog
in
      `HISTORY.md`
- [x] This PR is ready to go

CONTRIBUTING.md

@@ -189,46 +189,70 @@ Also, prefer to link to the paper's website, rather than just the PDF.
 
 ### Deploying
 
+We roughly follow the trunk-based release model described
+[here](https://trunkbaseddevelopment.com/branch-for-release/). Namely, we create
+release branches for minor versions (e.g., `release/0.6.x`) from our trunk
+(`master` branch). For patch releases, e.g., `0.6.2`, we cherry pick bug fixes
+from `master` into the corresponding release branch.
+
+#### Minor Versions
+
 1. Create a PR into master after doing the following:
-   1. Switch tutorial links from latest to stable with:
-      ```bash
-      make tutorial_links
-      ```
-      See [#300](https://github.com/icaros-usc/pyribs/pull/300) for why we do
-      this.
-   1. Update the version with `bump2version` by running the following for minor
-      versions:
+   1. Update the version with `bump2version`:
       ```bash
       bump2version minor
       ```
-      or the following for patch versions:
+   1. Add all necessary info on the version to `HISTORY.md`.
+1. Once the PR above has been merged, create a release branch from master called
+   `release/0.<NUM>.x`, e.g., `release/0.6.x`.
+1. On the release branch, switch tutorial links from latest to stable with:
+   ```bash
+   make tutorial_links
+   ```
+   See [#300](https://github.com/icaros-usc/pyribs/pull/300) for why we do this.
+1. (Optional) On the release branch, run `make release-test`. This uploads the
+   code to TestPyPI to check that the deployment works. If this fails, make
+   fixes as appropriate.
+1. Locally tag the head of the release branch, e.g.,
+   ```bash
+   git tag v0.6.0 HEAD
+   ```
+1. Now push the tag with
+   ```bash
+   git push --tags
+   ```
+1. Check that the version was deployed to PyPI. If it failed, delete the tag and
+   make appropriate fixes on master. Then, cherry pick the fixes into the
+   release branch. Finally, tag the HEAD again and push the tag.
+1. Write up the release on GitHub, and attach it to the tag.
+
+#### Patch Versions
+
+1. Create a PR into master after doing the following:
+   1. Update the version with `bump2version`:
       ```bash
       bump2version patch
       ```
    1. Add all necessary info on the version to `HISTORY.md`.
-1. (Optional) Once the PR has passed CI/CD and been squashed-and-merged into
-   master, check out the squash commit and locally run `make release-test`. This
-   uploads the code to TestPyPI to check that the deployment works. If this
-   fails, make fixes as appropriate.
-1. Once the PR in step 1 and any changes in step 2 have passed CI/CD and been
-   squashed-and-merged into master, locally tag the master branch with a tag
-   like `v0.2.1`, e.g.
+1. Once the PR above has been merged, checkout the release branch for the
+   corresponding minor version, e.g., for `0.6.2`, check out `release/0.6.0`.
+1. On the release branch, cherry-pick the commit for the PR you just created.
+   Also cherry pick any other bug fixes which need to be released in this patch.
+1. On the release branch, edit `HISTORY.md` to remove any irrelevant history,
+   e.g., if there are upcoming changes that will be included only in the next
+   version.
+1. (Optional) On the release branch, run `make release-test`. This uploads the
+   code to TestPyPI to check that the deployment works. If this fails, make
+   fixes as appropriate.
+1. Locally tag the head of the release branch, e.g.,
    ```bash
-   git tag v0.2.1 HEAD
+   git tag v0.6.2 HEAD
    ```
 1. Now push the tag with
    ```bash
    git push --tags
    ```
-1. Check that the version was deployed to PyPI. If it failed, delete the tag,
-   make appropriate fixes, and repeat steps 2 and 3.
+1. Check that the version was deployed to PyPI. If it failed, delete the tag and
+   make appropriate fixes on master. Then, cherry pick the fixes into the
+   release branch. Finally, tag the HEAD again and push the tag.
 1. Write up the release on GitHub, and attach it to the tag.
-1. Submit another PR which reverts the changes to the tutorial links.
-   Specifically, while on master, make sure your workspace is clean, then revert
-   the changes with:
-   ```bash
-   git checkout HEAD~ tutorials/
-   ```
-   And commit the result.
-
-Our deployment process may change in the future as pyribs becomes more complex.

HISTORY.md

@@ -1,21 +1,29 @@
 # History
 
-## (Forthcoming)
+## 0.7.0 (Forthcoming)
 
 ### Changelog
 
 #### API
 
 - Add GradientOperatorEmitter to support OMG-MEGA and OG-MAP-Elites ({pr}`348`)
 
-## 0.6.1
+## 0.6.2
 
 ### Changelog
 
 #### API
 
 - Import ribs[visualize] in tutorials that need it ({pr}`379`)
 
+#### Improvements
+
+- Switch to a branch-based release model ({pr}`382`)
+
+## 0.6.1
+
+(This release was removed)
+
 ## 0.6.0
 
 ### Changelog