added new changelog limitations (#5366)

* added new changelog limitations

* removed 'when possible'

* added explicit time window

.github/pull_request_template.md

@@ -33,7 +33,8 @@
 - [ ] My change requires a change to the documentation and I have updated the documentation accordingly.
 - [ ] I have added tests to cover my changes.
 - [ ] I have added myself to the [.zenodo.json](https://github.com/USGS-Astrogeology/ISIS3/blob/dev/.zenodo.json) document.
-- [ ] I have added any user impacting changes to the [CHANGELOG.md](https://github.com/USGS-Astrogeology/ISIS3/blob/dev/CHANGELOG.md) document.
+- [ ] I have added my user impacting change to the [CHANGELOG.md](https://github.com/USGS-Astrogeology/ISIS3/blob/dev/CHANGELOG.md) document. <!--- NOTE: You can only have one changelog entry per PR, see https://github.com/DOI-USGS/ISIS3/blob/dev/CONTRIBUTING.md -->
+
 
 ## Licensing
 This project is mostly composed of free and unencumbered software released into the public domain, and we are unlikely to accept contributions that are not also released into the public domain. Somewhere near the top of each file should have these words:

CONTRIBUTING.md

@@ -45,4 +45,7 @@ by discussion, compromise on the proposed change, or withdrawal of the proposed
 
 When making a bug fix contribution, please add the [bug](https://github.com/USGS-Astrogeology/ISIS3/pulls?q=is%3Apr+label%3Abug+) label to your pull request. This will automatically cherry-pick your PR into the latest release branch so that it can be distributed in the next bug fix release. 
 
+Every pull request making changes to ISIS's source needs a [changelog entry](https://raw.githubusercontent.com/DOI-USGS/ISIS3/dev/CHANGELOG.md). PRs are limited to one changelog entry per PR. If you have multiple bug fixes or feature adds, they should be split up. If PRs need to be merged in a particular order, include that as a comment on your PRs. If PR is one changelog, but high-volume changes such as large feature branches (many changes across many files requiring several hours of review time), then the PR isn't guaranteed to be reviewed promptly. Authors should contact the repo owners, see [code.json](https://github.com/DOI-USGS/ISIS3/blob/dev/code.json), to coordinate reviews outside typical USGS Astro support hours. We plan out our schedules in 6 week increments, so plan to coordinate with repo owners a minimum of 6 weeks before you expect the changes to be ready. 
+
 When making a contribution for the first time, please add your name to the `.zenodo.json file.` We strongly recommend adding your affiliation and ORCiD to the `zenodo.json` file. These additions only have to happen once.
+