Add text to compatible sources - aggregation

docs/data-provision/aggregation/compatible-sources.md

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-# OpenAIRE compatible sources
+# OpenAIRE compatible sources
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+The OpenAIRE aggregator collects metadata records from content providers compliant to the OpenAIRE guidelines.  
+
+The OpenAIRE Guidelines help repository managers expose publications, datasets and CRIS metadata via the OAI-PMH protocol in order to integrate with OpenAIRE infrastructure.
+
+You can find more information in https://guidelines.openaire.eu/en/latest/