* docs: mention PYTHONPATH in installing.rst
When pybind11 is included as a submodule, the user needs to update their
Python module search path. Otherwise, the first c++ compilation command
in docs/basics.rst will fail.
* docs: add a note about compiling the example
This note shows how to modify the compilation command for the example
when the pybind11 source has been included as a Git submodule.
* docs: add a note about compiling the example
Added an internal link to the docs
* docs: updated a note about compiling the example
Also updated the command substitution syntax for consistency
`git submodule add` needs the branch before the repository or else it is ignored. The previous code checked out the `master` branch, not the `stable` branch.
