docs(contributing): add a contributing guide

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+<!--- Describe your changes in detail -->
+
+## Related Issues
+
+- Closes #(add issue number)
+
+## Pre-launch Checklist
+
+- [ ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
+- [ ] I updated `pubspec.yaml` with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
+- [ ] I updated `CHANGELOG.md` to add a description of the change, [following repository CHANGELOG style].
+- [ ] I updated/added relevant documentation (doc comments with `///`).
+- [ ] All existing and new tests are passing.
+
+<!-- Links -->
+[Contributor Guide]: https://github.com/ricardodalarme/flutter_card_swiper/blob/main/CONTRIBUTING.md
+[pub versioning philosophy]: https://dart.dev/tools/pub/versioning
+[exempt from version changes]: https://github.com/ricardodalarme/flutter_card_swiper/blob/chore/add-a-contributing-guide/CONTRIBUTING.md#version
+[following repository CHANGELOG style]:https://github.com/ricardodalarme/flutter_card_swiper/blob/chore/add-a-contributing-guide/CONTRIBUTING.md#changelog
+
+## Visual reference
+
+<!--- Please include screenshots, gifs or recordings  -->
+<!--- For example: if this is a bug fix, provide before and after screenshots -->

CONTRIBUTING.md

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+# Contributing to this package
+
+## Table of Contents
+
+- [Contributing to this package](#contributing-to-this-package)
+  - [Table of Contents](#table-of-contents)
+  - [Version and CHANGELOG updates](#version-and-changelog-updates)
+    - [Version](#version)
+    - [CHANGELOG](#changelog)
+      - [Style](#style)
+      - [Example](#example)
+      - [Updating a CHANGELOG that has a NEXT](#updating-a-changelog-that-has-a-next)
+
+## Version and CHANGELOG updates
+
+### Version
+
+Any change that needs to be published in order to take effect must update the version in pubspec.yaml. There are very few exceptions:
+
+- PRs that only affect tests.
+- PRs that only affect example apps.
+- PRs that only affect local development (e.g., changes to ignored lints).
+- PRs that only make symbolic changes to the docs (e.g. typo).
+
+And the new version must follow the [pub versioning philosophy].
+
+### CHANGELOG
+
+All version changes must have an accompanying CHANGELOG update. Even version-exempt changes should usually update CHANGELOG by adding a special `NEXT` entry at the top of `CHANGELOG.md` (unless they only affect development of the package, such as test-only changes):
+
+```md
+## NEXT
+
+* Description of the change.
+
+## 1.0.2
+...
+```
+
+This policy exists because some changes (e.g., updates to examples) that do not need to be published may still be of interest to clients of a package.
+
+#### Style
+
+For consistency, all CHANGELOG entries should follow a common style:
+
+- Use `##` for the version line. A version line should have a blank line before and after it.
+- Use `-` for individual items.
+- Entries should use present tense indicative for verbs, with "this version" as an implied subject. For example, "Adds cool new feature.", not "Add" or "Added".
+- Entries should end with a `.`.
+- Breaking changes should be introduced with **BREAKING CHANGE**:, or **BREAKING CHANGES**: if there is a sub-list of changes.
+
+#### Example
+
+```md
+## 2.0.0
+
+- Adds the ability to fetch data from the future.
+- **BREAKING CHANGES**:
+  - Removes the deprecated `neverCallThis` method.
+  - URLs parameters are now `Uri`s rather than `String`s.
+
+## 1.0.3
+
+- Fixes a crash when the device teleports during a network operation.
+```
+
+#### Updating a CHANGELOG that has a NEXT
+
+If you are adding a version change to a CHANGELOG that starts with `NEXT`, and your change also doesn't require a version update, just add a description to the existing `NEXT` list:
+
+```md
+## NEXT
+
+* Description of your new change.
+* Existing entry.
+
+## 1.0.2
+...
+```
+
+If your change does require a version change, do the same, but then replace `NEXT` with the new version. For example:
+
+```md
+## 1.0.3
+
+* Description of your new change.
+* Existing entry.
+
+## 1.0.2
+...
+```
+
+<!-- Links -->
+[pub versioning philosophy]: https://dart.dev/tools/pub/versioning