Add validation

adding-validation.txt

application input: HttpInput (HTTP request body)                
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+--> wrap-json-body: Hash Map                      
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     +--> post-notes-handler: Hash Map        
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          +--> validate-note: [Hash Map, Error String] (present? correct length?)
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               +--> ok: [Hash Map] (HTTP response, status code 200)
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                    +--> wrap-json-response: JSON String
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                         +--> application output