Add the documentation on how to add custom meta tags through extra_metatags.module

As the module API is stable, I can now add a short description of how to add custom meta tags.
I will do it in the next minutes.

I am changing the report title to something more appropriate.

I added a description of how to add custom meta tags; the description helps to understand the basic steps necessary to add meta tags not handled by the current Nodewords modules.

Nodewords keep in cache the meta tag descriptors; that is the reason you need to clean the cache.
The module has a function to clear just its cache; I should find a way to make the call to that function automatic. I think that extra_metatags.module could check the change time of the files, and call the function when a file has been recently changed.

I will create a task report for that.

The code has it is implemented now doesn't allow to translate the strings used for the permission. Being this the case, I need to drop the support for custom meta tags as it is implemented now.

I will probably create an empty module that can be used to implement custom meta tags (as it is done with Token, which comes with tokenSTARTER.module). Doing so, it will be also possible to update the list of implemented meta tags by calling a nodewords.module function.

please, don't blame me but... i cannot find this documentation. How can be new meta tags added?

Thank you very much.