refactor readme files: in julia tests

tests/julia/README.md

@@ -0,0 +1,38 @@
+# Overview of tests for the julia buildpack
+
+## Tested configuration files
+
+- [`Project.toml`](https://repo2docker.readthedocs.io/en/latest/config_files.html#project-toml-install-a-julia-environment)
+- [`REQUIRE`](https://repo2docker.readthedocs.io/en/latest/config_files.html#require-install-a-julia-environment-legacy)
+- [`requirements.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#requirements-txt-install-a-python-environment)
+
+## Test folders
+
+### project
+
+- Tests use of a `Project.toml` file for Julia, using the repo2docker default
+  version of Julia as specified in `julia_project.py`.
+
+### project-1.0.2
+
+- Tests use of a `Project.toml` file for Julia, using a version of Julia
+  specified via `julia = "=1.0.2"` in `Project.toml`'s `[compat]` section.
+
+### require
+
+- Tests use of a `REQUIRE` file for Julia, using the repo2docker default version
+  of Julia as specified in `julia_require.py`. Note that this is default version
+  is currently 0.6.4!
+
+- Starting with Julia v0.7 and up, the package manager has changed, so this
+  tests that the Julia version below that can be installed correctly as well.
+
+### require-1-requirements-file
+
+- Tests use of a `REQUIRE` file for Julia, using a major version version
+  specification. Note that this major version specification is currently
+  resolving to a pinned minor and patch version as declared in
+  `julia_require.py`.
+
+- Test use of a `requirements.txt` file, where it is expected to be respected
+  alongside the `REQUIRE` file.