Explain parser behaviour and format rules

Explain some of the format rules and parser behaviour
in the README file.

This commit also fixes a minimal typo in the render-test-case
comments.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>

README.md

-Repository for Apertis tests cases.
+# Apertis Tests Cases and Renderer
+
+This repository contains all the Apertis test cases written using the YAML format
+defined at the `format/` directory.
+
+The repository also contains the `renderer-test-case` program, which is used to
+generate the test case pages in HTML format from the YAML files.
 
 ## Format Rules/Notes
 
+- The renderer heavily uses `lists` of strings to parse those into the different
+  HTML components. Each string in a list can be started with a special character
+  to get a specific format if it is supported by the YAML directive containing
+  such a string.
+
+  For the `pre-conditions`, `notes`, `expected`, `run.steps` (only manual
+  tests) and `post-conditions` directives, start a line with the following
+  characters to give the described format:
+
+    - `$`: Command line.
+    - `>`: Command output.
+    - `@`: Attach image.
+    - `~`: Add a web link.
+
+  For the `run.steps` field in the automated tests, only the following formatting
+  rules apply:
+
+    - `#`: Add a comment.
+    - Everything else is a command line.
+
 - Test cases with an `install.deps` directive will automatically add a
-  `pre conditions` section to the test case page with the list of packages to
+  `pre-conditions` section to the test case page with the list of packages to
   install specified in such a directive.
 
 ## Macro helpers
@@ -13,7 +39,7 @@ available: `macro_ostree_preconditions` , `macro_install_packages_preconditions`
 and `macro_modules_preconditions`. Please check the tests cases for example about
 how to use them.
 
-## Command
+## Renderer
 
 The main program is located inside the `renderer/` subdirectory and can be directly
 invoked from inside that directory:
@@ -44,3 +70,14 @@ $ ./render-test-case ../tests-cases/ -d apertis-tests-cases-v0.1/
 
 Now all the html tests cases should be available inside the
 `apertis-tests-cases-v0.1/` directory.
+
+### Renderer (parser) behaviour
+
+- The renderer parser will always abort with a failure if a mandatory field is
+  missing.
+
+- The renderer parser will abort with a failure for those fields set to values
+  with incorrect types.
+
+- The renderer parser will show warning messages for unrecognized values of
+  fields with multiple choices (for example, image-type, execution-type).

renderer/render-test-case

@@ -103,7 +103,7 @@ def get_template_values(testcase_data):
             if mv == 'expected':
                 template_values.update({ mv : parse_list(value) })
             else:
-                # Set also the prority_color
+                # Set the priority_color.
                 if mv == 'priority':
                     template_values.update({ 'priority_color' :
                                              priority_color(value) })