Update documentation for write support

[sadikovi]

This PR updates documentation and rustdoc with write support updates:

• Write support mentioning in the main README
• Rust documentation updates for write structs, traits and methods
• Rust documentation example updates to include write examples
• Minor changes and simplifications

[sadikovi]

@sunchao could you review this PR? Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 614

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 95.606%

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Files with Coverage Reduction	New Missed Lines	%
file/writer.rs	21	95.91%
<!--	Total:	21		-->

Totals
Change from base Build 613:	0.0%
Covered Lines:	12359
Relevant Lines:	12927

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💛 - Coveralls