Add filesystem library (#38)

* Add filesystem library

CPP17.md

@@ -26,6 +26,7 @@ C++17 includes the following new library features:
 - [std::invoke](#stdinvoke)
 - [std::apply](#stdapply)
 - [splicing for maps and sets](#splicing-for-maps-and-sets)
+- [std::filesystem](#stdfilesystem)
 
 ## C++17 Language Features
 
@@ -365,6 +366,22 @@ m.insert(std::move(e));
 // m == { { 1, "one" }, { 3, "three" }, { 4, "two" } }
 ```
 
+### std::filesystem
+The new `std::filesystem` library provides a standard way to manipulate files, directories, and paths in a filesystem.
+
+Here, a big file is copied to a temporary path if there is available space:
+```c++
+const auto bigFilePath {"bigFileToCopy"};
+if (std::filesystem::exists(bigFilePath)) {   
+  const auto bigFileSize {std::filesystem::file_size(bigFilePath)};
+  std::filesystem::path tmpPath {"/tmp"};
+  if (std::filesystem::space(tmpPath).available > bigFileSize) {
+    std::filesystem::create_directory(tmpPath.append("example"));
+    std::filesystem::copy_file(bigFilePath, tmpPath.append("newFile"));
+  }
+}
+```
+
 ## Acknowledgements
 * [cppreference](http://en.cppreference.com/w/cpp) - especially useful for finding examples and documentation of new library features.
 * [C++ Rvalue References Explained](http://thbecker.net/articles/rvalue_references/section_01.html) - a great introduction I used to understand rvalue references, perfect forwarding, and move semantics.