Bitcoin is a highly active project frequently releasing new releases. The current version at the time of writing is 0.15—you can view a list of project releases at https://bitcoin.org/en/version-history. From one version to another, a lot of features get added or removed. Therefore, you must examine the changelog to see whether there is any special feature you are interested in. For example, internal mining got removed in version 0.13.
To build Readercoin, I've opted for using the last major version of Bitcoin, that is, version 0.15 (You can also use the newer version 0.16). Create a directory called readercoin and clone the Bitcoin 0.15 code as follows:
git clone -b 0.15 https://github.com/bitcoin/bitcoin.git readercoin && ...