MAINT: Update PR template

As the guidelines are intended for the PR submitter, and not the reviewer, I added them to the comments, and changed the section name to "Pull request agreement".
Additionally, I made the final checklist item something everyone can check. This will avoid the "3 of 4" items complete indicator in the pull request list.

Author: effigies

docs/pull_request_template.md

@@ -15,11 +15,11 @@ of ongoing development efforts and possible overlaps between contributions).
 Please summarize here the main changes to the documentation that the reviewers should be aware of.
 -->
 
-## Pull-request guidelines:
+## Pull-request agreement:
 <!--
 Please make sure you have read the PR guidelines below, and checked 
 all boxes that apply at the bottom.
--->
+
 1. We invite you to list yourself as a *fMRIPrep* contributor, so if your name 
    is not already mentioned, please modify the 
    [``.zenodo.json``](https://github.com/poldracklab/fmriprep/blob/master/.zenodo.json)
@@ -55,13 +55,13 @@ all boxes that apply at the bottom.
    provided with this PR, within the PR or in future PRs.
    
 
-**Please review and check the following**:
-<!-- replace the empty checkboxes [ ] below with checked ones [x] accordingly -->
+Please review and check the following:
+(Replace the empty checkboxes [ ] below with checked ones [x] accordingly) -->
 
   - [ ] I have read the [guidelines for contributions](https://github.com/poldracklab/fmriprep/blob/master/CONTRIBUTING.md).
   - [ ] I understand that my contributions will not be merged unless the work is
         finished (i.e. no ``[WIP]`` tag remains in the title of my PR) and tests pass.
   - [ ] The proposed code follows the
         [coding style](https://github.com/poldracklab/fmriprep/blob/master/CONTRIBUTING.md#fmriprep-coding-style-guide),
         to the extent I understood them (and I will address any comments raised by the PR's reviewers in this regard).
-  - [ ] \(Optional\) I opt-out from being listed in the `.zenodo.json` file.
+  - [ ] I have added myself to the `.zenodo.json` file, OR I decline to be listed in the `.zenodo.json` file at this time.