docs: add appendix to explain special case.

docs/appendix.rst

@@ -0,0 +1,50 @@
+.. _appendix-ref:
+
+Appendix
+========
+
+Hint to avoid network errors
+----------------------------
+
+This section provide several hints, if you need to use aqtinstall in an environment that is limited to access the internet,
+or mirror server you redirected seems broken.
+You can find a description of the library to access network worked under the hood at first.
+We also provide solutions for several typical cases you may encounter.  
+
+Network access library
+----------------------
+
+The aqtinstall use `Requests`_ library to access the Qt binary archive repository provided by The Qt Foundation.
+The `Requests`_ is very famous and a popular library in python ecosystem, which has many tips and tutorials on the net.
+The aqtinstall has a special handler for HTTP direct, HTTP response code 30x, because the aqtinstall provides
+blacklisting of proxy urls.
+
+Zero trust network
+------------------
+
+The `Zero trust network` is popular administration concept in a enterprise local network.
+Enterprise administrator force employers to install network traffic redirector agent in PC and tablet.
+All the traffic are forwarded to cloud service that decrypt SSL communication and checks all the
+employers communications. Security vendor emphasis it as "SECURE" by zero trust for employers privacy.
+
+To realize it, all the communication between your PC to internet is encrypted with self-signed SSL certification.
+Administrator force installing the self-signed root certificate to your PC's Cert Manager.
+
+Python standard package manager utility PIP, and aqtinstall depends on ``certifi`` library for verification of SSL/TLS
+certifications, and no trust for the cert manager of Operating System.
+It will cause certification validation error when installing python packages, downloading Qt packages from the internet.
+
+The list of trusted CAs can also be specified through the ``REQUESTS_CA_BUNDLE`` environment variable. 
+You can check Requests manual section `SSL cert verification`_ for details.
+
+.. _`Requests`: https://requests.readthedocs.io/en/latest/
+.. _`SSL cert verification`: https://requests.readthedocs.io/en/latest/user/advanced/#ssl-cert-verification
+
+Got broken mirror
+-----------------
+
+When you got connection error with specific mirror site, when administrator of the mirror failed to configure the site,
+or the site policy limit the access in specific IP range, but you are not in it in any reason, but https://download.qt.io/
+mother site redirect you there.
+
+Please check :ref:`configuration-ref` section how to specify a blacklist of proxy site.

docs/index.rst

@@ -10,6 +10,7 @@ Contents:
    getting_started
    cli
    configuration
+   appendix
    CONTRIBUTE
    CODE_OF_CONDUCT
    SECURITY