Added diff -r command example and explanation in directories_comparison.md (bregman-arie#10596)

Author: mohitpant1010

exercises/shell/solutions/directories_comparision.md

@@ -0,0 +1,13 @@
+## How to compare two directories in Linux?
+
+You can use the 'diff' command with the '-r' flag to compare two direcotries recursively.
+
+
+
+### Example:
+'''bash
+diff -r folder1/ folder2/
+
+This command compares all the files and subdirectories inside 'folder1' and 'folder2'.
+If both directories have identical contents, it retuns nothing.
+If there are differences,it showss which files differ or are missing.