update release process documentation

Motivation

Added instructions for making the pre-release PR and going through the template checklist. I also moved some of the prerequisites to this new section.

Checklist

[x] Did you update CHANGELOG.md with your changes?
[X] Have you checked our Contributing document?
[X] Have you ensured the PR clearly describes the problem and the solution?
[x] Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
[x] Have you checked to ensure that there aren't other open Pull Requests for the same change?
[x] Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.91%. Comparing base (d3ac4b9) to head (db8fb40). Report is 17 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1928 +/- ## ======================================= Coverage 91.91% 91.91% ======================================= Files 27 27 Lines 2648 2648 Branches 691 691 ======================================= Hits 2434 2434 Misses 141 141 Partials 73 73 ``` | [Flag](https://app.codecov.io/gh/NeurodataWithoutBorders/pynwb/pull/1928/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/NeurodataWithoutBorders/pynwb/pull/1928/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | `72.80% <ø> (ø)` | | | [unit](https://app.codecov.io/gh/NeurodataWithoutBorders/pynwb/pull/1928/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | `83.98% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders#carryforward-flags-in-the-pull-request-comment) to find out more.

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NeurodataWithoutBorders / pynwb