[readme] add advanced example

README.md

@@ -1,6 +1,9 @@
 # progressbar
+
 A very simple, header-only, fully customizable, progress bar (with percentage)
 for c++ loops.
+
+Very simple to set up:
 ```cpp
 #include "progressbar.hpp"
 
@@ -15,7 +18,27 @@ int main() {
 ```
 ![animated gif](.github/example-simple.gif)
 
+Allows customization:
+```cpp
+#include "progressbar.hpp"
+
+int main() {
+    progressbar bar(100);
+    bar.set_todo_char(" ");
+    bar.set_done_char("█");
+    bar.set_opening_bracket_char("{");
+    bar.set_closing_bracket_char("}");
+    for (int i = 0; i < 100; ++i) {
+        bar.update();
+        // ... the program
+    }
+    return 0;
+}
+```
+![animated gif](.github/example-custom.gif)
+
 ## Notes
+
 To use the bar in parallelized loops call `progressbar::update` in a critical
 section. With [OpenMP](http://www.openmp.org) this can be achieved with the
 following structure: