added documentation on files backup

cs50.dev.md

@@ -104,6 +104,25 @@ cd Hello-World
 
 at which point you could use `git` within that `/workspaces/Hello-World` directory as usual. Note that only `/workspaces/$RepositoryName` will be automatically backed up to CS50's `code50` organization; repositories that you clone into `/workspaces` will not.
 
+### gitignore
+
+Currently, only these files will be backed up to your Codespace GitHub repository:
+
+- *.c
+- *.cpp
+- *.css
+- *.db
+- *.h
+- *.html
+- *.java
+- *.js
+- *.md
+- *.py
+- *.sql
+- *.ts
+- *.txt
+- Makefile
+
 ## GitHub
 
 ### Authorization