Add FAQ section to GitHub README

[jzabroski]

Example questions:

1. Why would I use OpenSpreadsheet over Microsoft's Open XML API?
2. Why would I use OpenSpreadsheet over ClosedXML? A: When the author tried using ClosedXML for his business purposes, it constantly ran out of memory when writing large datasets, and it seemed a lot of work to get it to write a simple IEnumerable<T>. (Would be good to show comparison code.) A: ClosedXML has some performance issues saving large sheets, due to slow GetHashCode implementations that allocate objects on the heap. A: ClosedXML has a memory leak where on consecutive runs of an open-edit-save workflow, memory balloons until the program crashes. https://github.com/ClosedXML/ClosedXML/issues/607
3. How do I write fluent tests with OpenSpreadsheet?
4. What runtimes does OpenSpreadsheet support?
5. This is a great idea. How can I help? -- suggest you request help for TravisCI automation or equivalent, so that you can add maintainers other than yourself going forward when life gets hard.