Document Ruby Tests (#145)

Add install prerequisites and test subsection Ruby Tests

Signed-off-by: Lucas Fernando <[email protected]>
Co-authored-by: Louise Poubel <[email protected]>
Signed-off-by: Louise Poubel <[email protected]>

README.md

@@ -90,6 +90,20 @@ The optional Eigen component of Ignition Math requires:
     sudo apt-get install libeigen3-dev
     ```
 
+The optional Ruby tests of Ignition Math require:
+
+ * [Ruby](https://www.ruby-lang.org/). Refer to the [Ruby Documentation](https://www.ruby-lang.org/downloads/) for installation instructions. On Ubuntu systems `apt-get` can be used to install Ubuntu Package `ruby-dev`:
+
+    ```
+    sudo apt-get install ruby-dev
+    ```
+
+  * [Swig](http://www.swig.org/). Refer to the [Swig Documentation](http://www.swig.org/download.html) for installation instructions. On Ubuntu systems `apt-get` can be used to install Swig:
+
+    ```
+    sudo apt-get install swig
+    ```
+
 ### Building from source
 
 1. Clone the repository
@@ -162,6 +176,28 @@ Follow these steps to run tests and static code analysis in your clone of this r
     make codecheck
     ```
 
+## Ruby Tests
+
+### Usage
+
+The C++ classes are available in Ruby code by interface files (`.i`) used by swig to build a C++ extension module.
+
+The interfaces and Ruby test codes are in the `src` folder. To use a C++ class in Ruby you need to:
+
+1. Create an interface file describing the class as in Swig and Ruby reference at [The Ruby-to-C/C++ Mapping](http://www.swig.org/Doc1.3/Ruby.html#Ruby_nn11)
+
+2. Include the interface file in `/src/ing_math.i`
+
+3. Create the Ruby file and import the class as in Swig and Ruby reference at [C++ Classes](http://www.swig.org/Doc1.3/Ruby.html#Ruby_nn18)
+
+### Tests
+
+`make test` already runs all tests, including the ones made in Ruby, but you can run a Ruby test individually using
+
+```
+ctest -R Ruby_TEST.rb
+```
+
 # Folder Structure
 
 Refer to the following table for information about important directories and files in this repository.