Add or update sample `readme.md` files

- #2731

samples/MvcMinimalSample.Web/readme.md

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+MvcMinimalSample.Web
+===
+This sample web project illustrates a minimal site using `Microsoft.AspNet.Mvc.Core`.
+Please build from root (`.\build.cmd` on Windows; `./build.sh` elsewhere) before using this site.

samples/MvcSample.Web/readme.md

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+MvcSample.Web
+===
+This sample web project illustrates various MVC 6 features. Please build from root
+(`.\build.cmd` on Windows; `./build.sh` elsewhere) before using this site.

samples/TagHelperSample.Web/readme.md

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+TagHelperSample.Web
+===
+This sample web project illustrates TagHelper use. Please build from root
+(`.\build.cmd` on Windows; `./build.sh` elsewhere) before using this site.