Added iterative GMRES example

[leespen1]

Added example of how to avoid repeat allocations when calling GMRES for multiple values of the right-hand side b, following the discussion in this discourse thread: https://discourse.julialang.org/t/iterativesolvers-jl-not-working-as-expected-with-mutating-and-allocation/110277/10.

I haven't been able to preview the result because I can't get Documenter to make the docs (I'm not super familiar with making documentation pages, and it keeps complaining about missing docs, even when I have cloned the master branch without any modifications), but since I am only adding to a markdown file I think it should work fine as long as everyone is satisfied with the content.

[coveralls]

Pull Request Test Coverage Report for Build 8163886000

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 97.46%

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Totals
Change from base Build 7903548867:	0.0%
Covered Lines:	2034
Relevant Lines:	2087

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💛 - Coveralls