[ENH] BIDS software development guidelines appendix

mkdocs.yml

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         - MEG file formats: 99-appendices/06-meg-file-formats.md
         - MEG systems: 99-appendices/07-meg-systems.md
         - Coordinate-systems: 99-appendices/08-coordinate-systems.md
+        - BIDS software development guidelines: 99-appendices/09-software-guidelines.md
       - Changelog: CHANGES.md
     - The BIDS Starter Kit: https://github.com/bids-standard/bids-starter-kit

src/99-appendices/09-software-guidelines.md

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+# Appendix IX: BIDS software development guidelines 
+
+This section RECOMMENDS some practices which could be adopted while developing
+BIDS compatible software.
+
+## Be consist with BIDS terminology
+
+[Common Principles: Definitions](../02-common-principles.md#definitions) and
+[Appendix IV: Entity table](04-entity-table.md) define some definitions and 
+terms which ideally be used in the UI of the tools to make tools easier to use
+by BIDS-aware users.
+
+## Be resilient and future-proofing
+
+BIDS is an evolving standard, and although we strive to clearly and unambiguously
+describe all entities and filenames convention, retain backward compatibility, 
+and version every release of the specification, some aspects could remain not
+clarified and open for interpretation or future change.  E.g., the set of 
+allowed characters in `<label>` might eventually change [ISSUE-226] or 
+[inheritance principle](../02-common-principles.md#the-inheritance-principle)
+could be further formalized (see [ISSUE-102]).
+It is RECOMMENDED that BIDS supporting applications are coded adhering 
+to [Postel's principle](https://en.wikipedia.org/wiki/Robustness_principle)
+which is driving TCP implementations to *"be conservative in what you do, be 
+liberal in what you accept from others."*
+
+
+[ISSUE-226]: https://github.com/bids-standard/bids-specification/issues/226
+[ISSUE-102]: https://github.com/bids-standard/bids-specification/issues/102