[Doc] Add code example of how to use custom filters

[zenpk]

Description

Yesterday I tried to use a custom filter but couldn't get it to work. I looked in the documentation and example code but found no information about this, so I read the source code and figured out that I had to call the FilterRule method. Upon reflection, this design makes sense and feels natural but I think it would be good to have an example in the readme.

Changes Made

1. Add a FilterRule call in the readme's Add Custom Filter section.
2. Uncomment a line in the example code, making the usage of the custom filter more obvious.

[coveralls]

Pull Request Test Coverage Report for Build 9827826902

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.68%

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Totals
Change from base Build 8995471607:	0.0%
Covered Lines:	2912
Relevant Lines:	3012

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💛 - Coveralls