Add README file to doc-generator tool (#7983)

**What this PR does / why we need it**:

Add README file to doc-generator tool

tools/doc-generator/README.md

@@ -0,0 +1,23 @@
+# Documentation generation tool
+
+The doc-generator tool is used to automatically generate the configuration flags documentation from the metadata information
+provided in the code. 
+
+## Run 
+
+The tool receives as input a template file and generates the list of configuration value blocks: 
+
+```shell
+go run ./tools/doc-generator docs/sources/configuration/index.template > docs/sources/configuration/_index.md
+```
+
+## `doc` tag
+
+The description and default value of configuration values can be set via CLI flag registration by using the `flag` package. 
+However, for a more flexible documentation generation it is possible to combine this with the `doc` tag by applying the following custom values:
+
+* `doc:"deprecated"`: sets the element as deprecated in the documentation.
+* `doc:"hidden"`: does not show the element in the documentation.
+* `doc:"description=foo"`: overrides the element's description (set via flag registration, if any) with `foo`.
+* `doc:"default=<hostname>"`: sets the element's documentation default value as `<hostname>`. 
+Note: this only sets the default value shown in the documentation, it doesn't override the default configuration value.