refactor readme files: in r tests

Author: consideRatio

tests/r/README.md

@@ -0,0 +1,43 @@
+# Overview of tests for the R buildpack
+
+## Tested configuration files
+
+- [`runtime.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#runtime-txt-specifying-runtimes).
+- [`DESCRIPTION`](https://repo2docker.readthedocs.io/en/latest/config_files.html#description-install-an-r-package).
+- [`install.R`](https://repo2docker.readthedocs.io/en/latest/config_files.html#install-r-install-an-r-rstudio-environment).
+- [`requirements.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#requirements-txt-install-a-python-environment)
+- [`apt.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#apt-txt-install-packages-with-apt-get)
+
+## Test folders
+
+### r-rspm-apt-file
+
+- Test setup of the default R environment by omitting a version specification in
+  `runtime.txt`, where the date provided in `runtime.txt` is recent enough for a
+  RSPM snapshot of CRAN to be used.
+
+- Test use of a `apt.txt` file.
+
+### r-rspm-description-file
+
+- Test use of a `DESCRIPTION` file instead of an `install.R` file, where a
+  `runtime.txt` is omitted and a recent enough snapshot date is assumed a RSPM
+  snapshot of CRAN to be used.
+
+### r3.6-mran
+
+- Test setup of a R 3.6 environment by specifying `r-3.6-...` in `runtime.txt`,
+  where the date provided in `runtime.txt` is old enough for a MRAN snapshot of
+  CRAN to be used.
+
+### r4.0-mran
+
+- Test setup of a R 4.0 environment by specifying `r-4.0-...` in `runtime.txt`,
+  where the date provided in `runtime.txt` is old enough for a MRAN snapshot of
+  CRAN to be used.
+
+### r4.0-rspm
+
+- Test setup of a R 4.0 environment by specifying `r-4.0-...` in `runtime.txt`,
+  where the date provided in `runtime.txt` is recent enough for a RSPM snapshot
+  of CRAN to be used.