Changelog update, minor template updates.

[PGijsbers]

Changes:

• Updates changelog with (hopefully) all changes since v0.8 (based on merged pull requests).
• Update the PR template to mention updating the changelog.
• Update the contribution guidelines to use Github's draft pull request instead of [WIP]/[MRG] tags.

[codecov-io]

Codecov Report

Merging #691 into develop will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## develop #691 +/- ##
========================================
Coverage 90.35% 90.35% ========================================
Files 36 36 Lines 3785 3785 ========================================
Hits 3420 3420 Misses 365 365

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