Add xeus to the documentation on how to write a kernel for Jupyter

Author: martinRenou

docs/kernels.rst

@@ -11,14 +11,25 @@ includes a kernel for Python code, and people have written kernels for
 At kernel startup, Jupyter passes the kernel a connection file. This specifies
 how to set up communications with the frontend.
 
-There are two options for writing a kernel:
+There are three options for writing a kernel:
 
 1. You can reuse the IPython kernel machinery to handle the communications, and
    just describe how to execute your code. This is much simpler if the target
    language can be driven from Python. See :doc:`wrapperkernels` for details.
 2. You can implement the kernel machinery in your target language. This is more
    work initially, but the people using your kernel might be more likely to
    contribute to it if it's in the language they know.
+3. You can use the `xeus <https://github.com/QuantStack/xeus>`_ library that is
+   a C++ implementation of the Jupyter kernel protocol. Kernel authors only need to
+   implement the language-specific logic in their implementation
+   (execute code, auto-completion...). This is the simplest
+   solution if your target language can be driven from C or C++: e.g. if it has
+   a C-API like most scripting languages. Check out the
+   `xeus documentation <https://xeus.readthedocs.io/>`_ for more details.
+   Examples of kernels based on xeus include:
+     - `xeus-cling <https://github.com/QuantStack/xeus-cling>`_
+     - `xeus-python <https://github.com/QuantStack/xeus-python>`_
+     - `JuniperKernel <https://github.com/JuniperKernel/JuniperKernel>`_
 
 Connection files
 ================