update to README.md and gdu.spec

[daniejstriata]

• Improved the Windows install steps.
• Updated the blurp about similar software.
• Added new RPM Spec file.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (1a763ae) 93.58% compared to head (12bfc6a) 93.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #281 +/- ## ======================================= Coverage 93.58% 93.58% ======================================= Files 29 29 Lines 2806 2806 ======================================= Hits 2626 2626 Misses 139 139 Partials 41 41 ```

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