Added RPi cheatsheets (#5696)

* add to documentation

* Added RPi cheatsheets

* Update free-programming-cheatsheets.md

* Update free-programming-cheatsheets.md

* Update free-programming-cheatsheets.md

* Update free-programming-cheatsheets.md

* Update free-programming-cheatsheets.md

fixed 404 broken link issue

* Update free-programming-cheatsheets.md

* Update CONTRIBUTING.md

* Added Blockchain books -PDF

* Delete free-programming-books-langs.md

* Re-added books

* Additional changes, mods.

* Update free-programming-books-langs.md

* Modifications contrib.

* Update CONTRIBUTING.md

* Update CONTRIBUTING.md

* Additions for Tensorflow cheatsheets (other branch)

* Update free-programming-cheatsheets.md

* Changes on other files

* Update free-programming-cheatsheets.md

* Additional changes

* Final changes for cheatsheets page nl ends.

* Update free-programming-cheatsheets.md

added the official content from kaggle for TF (includes pdf link, html only)
removed the blog TF link at the top (as per contributing.md guidelines)

* Update free-programming-books-langs.md

About: Blockchain additions, removed all the whitepapers from the list.

Why is the "Introducting Eth*" allowed in the PDF list of ebooks and the "*for dummies" series  not allowed? I do not see that rule in the contributing.md file
AFAIK, both are online ebooks links, both with an ISBN/Table Of Contents for chapters.

Either way, i replaced the first one with one that is free from offic. ibm public link

* delete patch

* Remove patch. not free

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* use wayback machine to provide its backed PDF link

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* add in process, creator

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* blockchain is not a language. the dummies book should be is subjects list

Co-authored-by: eric <eric@hellman.net>
Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

more/free-programming-cheatsheets.md

@@ -25,10 +25,12 @@
 * [Perl](#perl)
 * [Python](#python)
 * [R](#r)
+* [Raspberry Pi](#raspberry-pi)
 * [Ruby](#ruby)
 * [Rust](#rust)
 * [Solidity](#solidity)
 * [SQL](#sql)
+* [Tensorflow](#tensorflow)
 
 
 ### Artificial Intelligence
@@ -188,6 +190,15 @@
 * [All RStudio cheatsheets resources](https://www.rstudio.com/resources/cheatsheets) - RStudio.com (HTML site with PDF links)
 
 
+### Raspberry Pi
+
+* [Basic GPIO layout configuration cheatsheet](https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/robot/cheat_sheet/) - University of Cambridge Computer Laboratory Raspberry Pi Projects Cheatsheet (PDF)
+* [Other Raspberry Pi Commands cheatsheet](https://www.raspberrypistarterkits.com/wp-content/uploads/2018/01/raspberry-pi-commands-cheat-sheet.pdf) - RPi starter Kit (PDF)
+* [Raspberry Pi Basics cheatsheet](https://www.woolseyworkshop.com/wp-content/uploads/WoolseyWorkshop_Cheatsheet_RaspberryPiBasics_v1.4.pdf) - Woolsey Workshop (PDF)
+* [Raspberry Pi Different GPIO Configuration Combinations cheatsheet](https://static.raspberrypi.org/files/education/posters/GPIO_Zero_Cheatsheet.pdf) - GPIO Zero Cheatsheet (PDF)
+* [Top 50 General Commands for Raspberry Pi cheatsheet](https://projects-raspberry.com/wp-content/uploads/2018/05/Top-50-Raspberry-pi-commands-List-cheat-sheet.pdf) - Projects Raspberry (PDF)
+
+
 ### Ruby
 
 * [Ruby Cheat Sheet](https://www.codeconquest.com/wp-content/uploads/Ruby-Cheat-Sheet-by-CodeConquestDOTcom.pdf) - CodeConquest.com (PDF)
@@ -208,4 +219,10 @@
 
 * [MySQL Cheatsheet](https://s3-us-west-2.amazonaws.com/dbshostedfiles/dbs/sql_cheat_sheet_mysql.pdf) - Database Star (PDF)
 * [PostgreSQL Cheatsheet](https://s3-us-west-2.amazonaws.com/dbshostedfiles/dbs/sql_cheat_sheet_pgsql.pdf) - Database Star (PDF)
-* [SQL Cheatsheet](https://www.codecademy.com/learn/learn-sql/modules/learn-sql-manipulation/cheatsheet) - Codecademy (SQL)
+
+
+### TensorFlow
+
+* [DeepLearning- Keras & TF Cheat Sheet](https://cheatography.com/chesterhsieh/cheat-sheets/deeplearning-keras-and-tf/pdf/) - Chester Hsieh, Cheatography (PDF) *(:construction: in process)*
+* [TensorFlow Quick Reference Table](https://secretdatascientist.com/tensor-flow-cheat-sheet/) - Secret data scientist (HTML)
+* [TensorFlow v2.0 Cheat Sheet](https://web.archive.org/web/20200922212358/https://www.aicheatsheets.com/static/pdfs/tensorflow_v_2.0.pdf) - Altoros (PDF) *(:card_file_box: archived)*