Add notes about recipes for R packages (#4335)

* Add notes about recipes for R packages

Co-authored-by: Isabel Paredes <isabel.paredes@quantstack.net>

* Improve documentaton of packaging R packages

by incorporating suggestions from Isabel Paredes.

---------

Co-authored-by: Isabel Paredes <isabel.paredes@quantstack.net>

Author: rgaiacs

docs/development/adding_packages.md

@@ -144,3 +144,39 @@ requirements:
 * [cryptography](https://github.com/emscripten-forge/recipes/tree/main/recipes/recipes_emscripten/cryptography)
 * [pydantic-core](https://github.com/emscripten-forge/recipes/tree/main/recipes/recipes_emscripten/cryptography)
 * [pycrdt](https://github.com/emscripten-forge/recipes/tree/main/recipes/recipes_emscripten/pycrdt)
+
+## R Packages
+
+For a package without dependencies of other R packages, only these requirements are usually necessary:
+
+```yaml
+requirements:
+  build:
+  - cross-r-base_${{ target_platform }}
+  - ${{ compiler("c") }}
+```
+
+Some R packages use the [Rcpp] package for a seamless integration of R and C++. [Rcpp] and all other R-dependencies must be in all 3 `requirements` sections (`build`, `host`, and `run`). For example,
+
+```yaml
+requirements:
+  build:
+  - cross-r-base_${{ target_platform }}
+  - ${{ compiler("c") }}
+  - r-rcpp
+  host:
+  - r-rcpp
+  run:
+  - r-rcpp
+```
+
+There needs to be 2 urls because very frequently the first one is not reachable.
+
+For the tests, we only check that the shared library for the package exists. Eventually we should be able to load the package and do actual tests, but that's not available yet.
+
+**Example recipes**:
+
+* [r-yaml](https://github.com/emscripten-forge/recipes/blob/main/recipes/recipes_emscripten/r-yaml/)
+* [r-plyr](https://github.com/emscripten-forge/recipes/blob/main/recipes/recipes_emscripten/r-plyr/)
+
+[Rcpp]: https://cran.r-project.org/web/packages/Rcpp/index.html