Mention Dependency Injection support in Readme or in Wiki

[MNF]

In https://github.com/mganss/HtmlSanitizer#usage or in https://github.com/mganss/HtmlSanitizer/wiki/Getting-Started#usage it will be good to mention that IHtmlSanitizer supports Dependency Injection and can be used as a singleton, e.g.

For example, if you are using asp.net core, in Startup AddSingleton services.AddSingleton<IHtmlSanitizer>(_ => new HtmlSanitizer());

(From https://stackoverflow.com/questions/58065590/htmlsanitizer-asp-net-core-2-with-di)

and use constructor dependency injection to pass htmlSanitizer to your class

[mganss]

I've added a paragraph to the FAQ page in the Wiki: https://github.com/mganss/HtmlSanitizer/wiki/Frequently-Asked-Questions

[michael-freidgeim-webjet]

@mganss according to https://stackoverflow.com/questions/58065590/htmlsanitizer-asp-net-core-2-with-di he .Net framework dependency injection is trying to inject the optional constructor parameters which results in empty AllowedTags and AllowAttributes collections. Is the issue fixed? If not, explicit default constructor implementation is required. services.AddSingleton(_ => new HtmlSanitizer());

[mganss]

Yes, the issue was fixed in #370