Added documentation.

documentation/dev/pkgtest.md

+# PACKAGE TEST DOCUMENTATION
+
+## INSTALLED PACKAGES CACHE
+
+### Description
+
+Avoids re-installing of packages for every test. Packages are cached for a specific native API version, i.e., checksum of the native header files.
+
+Directory structure:
+- pkg-cache-dir
+--+- version.table
+--+- libraryVERSION0
+----+- pkgdir0
+----+- pkgdir1
+----+- ...
+--+- libraryVERSION1
+----+- pkgdir0
+----+- pkgdir1
+----+- ...
+--+- ...
+
+The API checksum must be provided because we do not want to rely on some R package to compute it.
+
+### Usage
+
+Run `mx pkgtest --cache-pkgs version=<checksum>,dir=<pkg-cache-dir>,size=<cache-size>,link=<TRUE|FALSE>`, e.g.
+```
+mx pkgtest --cache-pkgs version=730e109bd7a8a32b1cb9d9a09aa2325d2430587ddbc0c38bad911525,dir=/tmp/cache_dir
+```
+
+The `version` key specifies the API version to use, i.e., a checksum of the header files of the native API (mandatory, no default).  
+The `pkg-cache-dir` key specifies the directory of the cache (mandatory, no default).  
+The `size` key specifies the number of API version for which to cache packages (optional, default=`2L`).  
+The `link` key specifies is the number of API version for which to cache packages (optional, default=`FALSE`).  
+
+### Details
+
+The version must be provided externally such that the R script does not rely on any package.
+The version must reflect the native API in the sense that if two R runtimes have the same native API version, then the packages can be used for both runtimes.
+