[ADD] roadmap for scaleup tax shelter report

l10n_be_cooperator/readme/ROADMAP.md

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+**Tax shelter :**
+
+According to [this](https://eservices.minfin.fgov.be/myminfin-web/pages/public/fisconet/document/7049b083-3ae7-4250-bb35-c00b9130fb02#_VI._Documents_justificatifs) administration FAQ on tax shelter for scaleups (translated to english):
+
+>31. A royal decree (to be issued) will determine the manner in which evidence is to be provided and that the proof that the company in growth fulfills the criterion of growth of turnover or full-time equivalents.
+
+In the attestation template from the administration, there is currently some information on annual sales and number of employee for the past two years before the fiscal year. 
+It's a lot of information to add to the declaration. Since the legal situation seems still uncertain, we might wait before adding this information in the report.