Make throwing warnings on new checkout

[timvandenhof]

Running the Make command completes with errors about missing files. These files are relative to this repository and not part of this repository.

Reproduction

1. Make sure the mentioned pip modules are installed conform readme.
2. Clone/checkout master.
3. Run make in repository root folder.

Warnings

WARNING: Problems with "include" directive path: InputError: [Errno 2] No such file or directory: '../server/src/servermanager.cpp'.
WARNING: Include file '/Users/USERNAME/Documents/GitHub/tools/dumpbeaconsbin/dumpbeaconsbin.py' not found or reading it failed
WARNING: Problems with "include" directive path: InputError: [Errno 2] No such file or directory: '../client/scripts/shoppingdatatypes.cpp'.
WARNING: Include file '/Users/USERNAME/Documents/GitHub/tools/dumpshopbin/dumpshopbin.py' not found or reading it failed

Full console output

Running Sphinx v3.5.2
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 0 source files that are out of date
updating environment: 0 added, 2 changed, 0 removed
reading sources... [100%] modding/http-shopping                                 
/Users/USERNAME/Documents/GitHub/boundless.docs/modding/http-beacons.rst:32: WARNING: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: '../server/src/servermanager.cpp'.
/Users/USERNAME/Documents/GitHub/boundless.docs/modding/http-beacons.rst:49: WARNING: Include file '/Users/USERNAME/Documents/GitHub/tools/dumpbeaconsbin/dumpbeaconsbin.py' not found or reading it failed
/Users/USERNAME/Documents/GitHub/boundless.docs/modding/http-shopping.rst:48: WARNING: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: '../client/scripts/shoppingdatatypes.cpp'.
/Users/USERNAME/Documents/GitHub/boundless.docs/modding/http-shopping.rst:80: WARNING: Include file '/Users/USERNAME/Documents/GitHub/tools/dumpshopbin/dumpshopbin.py' not found or reading it failed
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] modding/http-shopping                                  
generating indices... genindex done
writing additional pages... search done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 4 warnings.

The HTML pages are in docs.

Maybe it is possible to create a contributing.md describing if it is desired to contribute and if so, what is required to make contributing possible.