Add more resources

[akshatvg]

We would love to add any more resources you have in mind. A few suggestions are:

• Big Data
• OS
• DBMS
• Anything you think will make a difference.

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[jugaldb]

OS resources Added.
Refer - #17

[lio2011]

Design resources added

[lio2011]

Security resources added too

[gupta-niharika]

added typeface resource for design #15

[mayankparida]

Hey, could you allow me to add a section for Flutter App Development as well?

[akshatvg]

Hey @mayankparida!

The section already exists here.

Feel free to add any cool new resources or links we might have missed here.