Contributor documentation directory and README (#383)

* documentation directory setup

* basic README.md file as introduction

* fix typos...

Co-authored-by: Dmitrii Orlov <orlov@satelligence.com>

contributor_docs/README.md

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+# Hello
+
+Welcome to the contributor's documentation section of `pypiserver`.  
+Here you can find information about the inner working of the system.  
+Contributions and support in maintaining (both the project and these documents)
+is very welcome :v:  
+
+> This documentation is still a work in progress and would benefit from
+    your help!  
+> Some areas to describe are highlighted in
+    [this issue](https://github.com/pypiserver/pypiserver/issues/368),
+    but feel free to add if you see something missing there.  
+> All the help will be very appreciated! :)  
+
+## Find more information
+
+If something is missing in the documentation here, try checking in the
+`README.rst` and if it's still not there, don't hesitate to contribute.  
+
+## Documentation organization
+
+This folder contains various documents describing the `pypiserver` that might
+be of interest to someone studying it in more details.  
+The documentation files are written in `.md` and it will be great if new
+files are added in the same format.  

contributor_docs/architecture_overview.md

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+# Architecture Overview
+
+To be described...