Added agency-index.md

Signed-off-by: Natalia Luzuriaga <natalia.luzuriaga@cms.hhs.gov>

Author: natalialuzuriaga

docs/agency-index.md

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+## agency-index.json
+
+The SHARE IT Act requires federal agencies to publish metadata for their software projects. This metadata is compiled into an **agency-index.json** file, which serves as a centralized inventory of the agency's software portfolio.
+
+
+Agency index files are found here: https://github.com/DSACMS/code-gov/tree/main/agency-indexes