Add a link to Git cheatsheet PDF in docs readme

[ci skip]

doc/README.md

@@ -21,6 +21,7 @@
 - [Workflow](workflow/README.md) Using GitLab functionality and importing projects from GitHub and SVN.
 - [University](university/README.md) Learn Git and GitLab through videos and courses.
 - [Git Attributes](user/project/git_attributes.md) Managing Git attributes using a `.gitattributes` file.
+- [Git cheatsheet](https://gitlab.com/gitlab-com/marketing/raw/master/design/print/git-cheatsheet/print-pdf/git-cheatsheet.pdf) Download a PDF describing the most used Git operations.
 
 ## Administrator documentation