[doc] Python sys.path

docs/how_to_add_custom_syspath.md

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+# How to add a custom syspath for Python
+
+When importing libraries, Python will check in the sys.path to know which
+subdirectories might contain a package. It will mark any directory containing a
+`__init__.py` file as a package or subpackage if its parent also has one.
+
+In order to create a package you then simply have to a have a structure like:
+
+```
+sound/                          Top-level package
+      __init__.py               Initialize the sound package
+      formats/                  Subpackage for file format conversions
+              __init__.py
+              wavread.py
+              wavwrite.py
+              aiffread.py
+              aiffwrite.py
+              auread.py
+              auwrite.py
+              ...
+      effects/                  Subpackage for sound effects
+              __init__.py
+              echo.py
+              surround.py
+              reverse.py
+              ...
+      filters/                  Subpackage for filters
+              __init__.py
+              equalizer.py
+              vocoder.py
+              karaoke.py
+              ...
+```
+
+And to be able to import this package from anywhere you can simply add the
+absolute path to `sound` to your syspath before importing it.
+
+## Method 1
+
+```python
+import sys
+import os
+
+# if you maximum priority you can add it first
+sys.path.insert(0, os.path.abspath('/path/to/your/package/'))
+
+# of add it at the end if you does not want priority
+sys.path.append(os.path.abspath('/path/to/your/package/'))
+```
+
+## Method 2
+
+You can use the `PYTHONPATH` env variable to add new paths to your syspath like
+so:
+```shell
+$ PYTHONPATH="/path/to/your/package/" python3
+```