Doc: Add documentation to explain how to create a new Joplin-specific package

readme/dev/adding_new_package.md

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+# Adding a new Joplin package
+
+To add a new Joplin package (in the `/packages` folder), several factors must be considered. Please make sure you follow this documentation to ensure the package can be correctly deployed and maintained by the existing tooling.
+
+In this documentation, replace `PACKAGE_NAME` by the name of the newly created package.
+
+## In `package.json`
+
+In the new package `package.json` file:
+
+- Set the `name` to `@joplin/PACKAGE_NAME`
+
+- Add `"publishConfig": { "access": "public" }`. Otherwise npm will try to publish the package as a private one, but we only want public packages. Also Lerna will fail when publishing the package if it is private since our repository is a free one (private packages are not supported).
+
+- Set the `version` to the current major/minor version of Joplin.
+
+- `license` should most likely be `AGPL-3.0-or-later`
+
+- `repository` should be `https://github.com/laurent22/joplin/tree/dev/packages/PACKAGE_NAME`
+
+## In `setupNewRelease.ts`
+
+Add a line to ensure that the version number is automatically updated:
+
+```typescript
+await updatePackageVersion(`${rootDir}/packages/PACKAGE_NAME/package.json`, majorMinorVersion, options);
+```
+
+## In `.npmpackagejsonlintrc.json`
+
+Add the name of the new packages to the exceptions. This is because Lerna set the package version number to something like `^x.y.z`, while we only support pinned versions `x.y.z`. But this is fine for Joplin-specific packages.
+
+## In `packages/app-mobile/metro.config.js`
+
+If the package is to be used by the mobile app, make sure you add it to the `localPackages` list:
+
+```javascript
+'@joplin/PACKAGE_NAME': path.resolve(__dirname, '../PACKAGE_NAME/'),
+```