Add .contains example to doc

[pyramid3d]

In the readme STL-like access section, there already is:

// find an entry
if (o.find("foo") != o.end()) {
  // there is an entry with key "foo"
}

Sometimes we just want to check if an entry is there.
For this .contains() is so much easier to read.
Please do add this example too:

// check if entry exists
if (o.contains("foo")) {
  // there is an entry with key "foo"
}

On a side note: great work! This library is facile to use and makes one's programming life much easier and much more fun! thanks for this library and your effort!

[pyramid3d]

Great! :-)