Merge pull request #43 from inexxt/Nonfunctional_docs

Non-functional requirements

docs/Nonfunctional.md

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+# Non-functional requirements
+
+Fault tolerance
+ - the application should be resiliant to errors caused by server-side disconnect (gracefuly show the error to the user)
+ - the application should be resiliant to errors caused by database connection errors (gracefuly show the error to the user)
+
+Open source
+ - the application and all of its components should be open-source
+
+Internationalization and localization
+ - the application should support timezones
+
+Backup:
+ - all data kept at one place (one-file database) to facilitate easy backup creation 
+
+Performance 
+ - the application opens in less than 10s
+ - adding/deleting events takes no more than 3s
+
+Platform portability
+ - the application should run on, at least: Windows 10 and Ubuntu Linux
+ - the development environment should be available on at least: Windows 10 and Ubuntu Linux
+
+Data portability 
+ - the user should be able to move the data out of the application easy (soft requirement)
+ - this means using an established, open-source data storage format (such as SQLite format)
+
+Ergonomy
+ - every main action in the program (calendar view, adding events, adding tasks) should be accessible from the main page
+ - the application should look the same on all platforms
+
+Volume
+ - the application should be able to handle large volume of user-generated data
+ - this means no errors handling at least 100 events for 1000 days
+
+Interoperability
+ - the application should support interoperability with other calendar software (using open format iCal)