[DOCS] Add README to Yaml Runner

Author: picandocodigo

elasticsearch-api/spec/README.md

@@ -0,0 +1,61 @@
+# Rest API YAML Spec Runner
+
+The file that traverses the yaml files and loads a **TestFile** object per each of them:
+`elasticsearch-api/spec/elasticsearch/api/rest_api_yaml_spec.rb`
+
+You can use the SINGLE_TEST env variable to run just one test, or add code like this on the first line of the tests.each block:  
+```ruby
+next unless file =~ /indices.put_mapping\/all_path_options_with_types.yml/
+```
+
+#### TestFile object
+Class representing a single test file. Contains setup, teardown and tests.   
+`../api-spec-testing/test_file.rb`
+
+#### Test object
+Every single test in the test file is represented in the Test object.   
+`../api-spec-testing/test_file/test.rb`
+
+#### TaskGroup objects
+
+Tests are ordered in task groups, an array of TaskGroup objects.  
+`../api-spec-testing/test_file/task_group.rb`  
+
+Task Groups are a representation of a block of actions consisting of 'do' actions and their verifications. e.g.: 
+```yaml
+ - do:
+      index:
+          index:  test-index
+          id:     1
+          body:   { foo: bar }
+
+ - match:   { _index:   test-index }
+ - match:   { _id:      "1"}
+ - match:   { _version: 1}
+```
+
+**Before** each test, the spec runner runs `clear_data` on the test_file. This clears indices, index templates, snapshots and repositories. For xpack it also clears roles, users, privileges, datafeeds, ml_jobs and more.
+
+**After** each test, it runs the test file teardown and `clear_data` again.
+
+For each TaskGroup, it sees what's in the task group definition and runs an expectation test.
+
+# Rest YAML tests Helper
+
+`elasticsearch-api/spec/rest_yaml_tests_helper.rb`
+
+- `ADMIN_CLIENT` is defined here.
+- `SINGLE_TEST` is defined here.
+- Skipped tests are listed here
+
+# Spec Helper
+
+- `DEFAULT_CLIENT` is defined here
+
+# RSpec Matchers
+
+The tests use custom [RSpec Matchers](https://www.rubydoc.info/gems/rspec-expectations/RSpec/Matchers) defined in `api-spec-testing/rspec_matchers.rb`.
+
+# Enable Logging
+
+To enable logging, set the environment `QUIET` to false before running the tests. In CI, this is located in the [Dockerfile](https://github.com/elastic/elasticsearch-ruby/blob/master/.ci/Dockerfile). The environment variable is evaluated in the Rest YAML tests Helper file.