Update: Meanwhile you could also switch to the BoringSSL fork.

D. J. Bernstein’s ChaCha20-Poly1305 has not been merged into the OpenSSL master branch yet (ETA, anyone?). If you are curious to test it with nginx or any other application relying on the OpenSSL libraries with support for TLS 1.2, you can check it out via the 1.0.2-aead branch:

$ git clone https://github.com/openssl/openssl.git
$ cd openssl
$ git checkout 1.0.2-aead

Then follow the usual instruction from the INSTALL file on how to compile OpenSSL.