Add documentation for the useCaseSensitiveFileNames option (#1227)

Setting this option on projects with lots of files can reduce compilation time. Case-insensitivty requires normalization of file paths, which involves executing regex's. This shows up in the hot path of compilation profiling.

README.md

@@ -62,6 +62,7 @@
     + [appendTsSuffixTo](#appendtssuffixto)
     + [appendTsxSuffixTo](#appendtsxsuffixto)
     + [onlyCompileBundledFiles](#onlycompilebundledfiles)
+    + [useCaseSensitiveFileNames](#useCaseSensitiveFileNames)
     + [allowTsInNodeModules](#allowtsinnodemodules)
     + [context](#context)
     + [experimentalFileCaching](#experimentalfilecaching)
@@ -625,6 +626,15 @@ loading only those files that are actually bundled by webpack, as well as any `.
 by the `tsconfig.json` settings. `.d.ts` files are still included because they may be needed for
 compilation without being explicitly imported, and therefore not picked up by webpack.
 
+#### useCaseSensitiveFileNames
+| Type | Default Value |
+|------|--------------|
+| `boolean` | determined by typescript based on platform |
+
+The default behavior of `ts-loader` is to act as a drop-in replacement for the `tsc` command,
+so it respects the `useCaseSensitiveFileNames` set internally by typescript. The `useCaseSensitiveFileNames` option modifies this behavior,
+by changing the way in which ts-loader resolves file paths to compile. Setting this to true can have some performance benefits due to simplifying the file resolution codepath.
+
 #### allowTsInNodeModules
 | Type | Default Value |
 |------|--------------|