Example on how differential IK may converge to a local constrained optimum

[stephane-caron]

Thanks to @JonathanKuelz for starting the discussion in https://github.com/stephane-caron/pink/discussions/66 and @devbanu for participating. Hoping these changes help clarify this for future travelers:

• Added an example to illustrate local sub-optimal convergence
• Added a section to the readme to highlight the distinction between local and global IK

[coveralls]

Pull Request Test Coverage Report for Build 7630383398

• 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 99.823%

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Totals
Change from base Build 7615682938:	0.0%
Covered Lines:	1128
Relevant Lines:	1130

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💛 - Coveralls