Explain what IMI is and link to its homepage

[bvhme]

Outsiders that would like to learn from this guide need to know the context of its goals, for this it is important to understand the role of IMI in the context of the Barcelona City Council.

[imingueza]

Pull request considered, modifications done in:

• https://github.com/AjuntamentdeBarcelona/eds-glossary-bcn-en/commit/e90d91911566a3b3c7af6f7604a0899b6276946f - New IMI glossary entry added.
• https://github.com/AjuntamentdeBarcelona/free-software-guidelines-bcn-en/commit/2842d927eea8f3c917f60c5863c1310d0542125d - Improved the short IMI explanation

In addition, we expect in the future that all entries that appears in the glossary, appears like a link automatically in all the references in the documents, so if "IMI" appears anywhere, only placing cursor over it or clicking, you could navigate until the definition.