doc(package): add basic README for package folder

package/README.md

@@ -0,0 +1,45 @@
+# GeoNetwork-UI NPM Package
+
+## Generating the package
+
+This folder contains the system for generating the [`geonetwork-ui` NPM package](https://www.npmjs.com/package/geonetwork-ui).
+
+It contains:
+* a `generate-package.js` node script
+* an `index.ts` file which serves as an entrypoint for the package compilation
+* a `ng-package.json` file which is used by [`ng-packagr`](https://github.com/ng-packagr/ng-packagr)
+* a `package.json` describing the NPM package
+* a `tsconfig.json` file used for the package compilation
+
+The `generate-package.js` file does:
+* copy the contents of the `libs` folder in the `package` directory, keeping only relevant files (ts, css, html...)
+* transform the import aliases in the TS files back to a relative path using the `paths` property of `tsconfig.base.json`; for instance:
+  ```ts
+  import { UiLayoutModule } from "@geonetwork-ui/ui/layout"
+  ```
+  becomes:
+  ```ts
+  import { UiLayoutModule } from "../../../libs/ui/layout"
+  ```
+  (this is needed because [`ng-packagr` does not support Typescript path aliases](https://github.com/ng-packagr/ng-packagr/pull/1502#issuecomment-572079617))
+* launch a compilation of the copied sources using [`ng-packagr`](https://github.com/ng-packagr/ng-packagr); this is necessary because Angular libraries have to be published according to the [Angular Package Format](https://angular.io/guide/angular-package-format)
+* the compilation output is going to `package/dist`; then the script also copies the source `libs` folder there as well so that consumers of the package can be redirected to the source file when inspecting it
+
+To generate the package, simply run:
+
+```shell
+node package/generate-package.js
+```
+
+Then the package can be published like so, assuming the correct rights are available:
+
+```shell
+cd package/dist
+npm publish
+```
+
+## Using the package
+
+Using the package requires creating a custom GeoNetwork-UI application.
+
+[See the documentation website for more information.](https://geonetwork.github.io/geonetwork-ui/main/docs/guide/custom-app.html)