Changing cd.txt to cd.md (Solving Issue#17)

cd/cd.md

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+# cd
+
+## Overview
+`cd` is a Unix command to move a directory. It will move to the path specified by absolute or relative path.
+
+## Syntax
+```
+cd [Folder Name]
+```
+## Different functionalities of cd command : 
+
+**cd /**: This command is used to change directory to the root directory, The root directory is the first directory in your filesystem hierarchy. 
+ ``` 
+  cd /
+ ```
+
+**cd dir_1/dir_2/dir_3**: This command is used to move inside a directory from a directory 
+
+ ```
+ cd dir_1/dir_2/dir_3
+ ```
+
+__cd__~ : This command is used to change directory to the home directory. 
+ ```
+ cd ~
+ ```
+
+**cd**: this command also works the same as the cd ~ command. 
+ ```
+ cd
+ ```
+
+**cd “dir name”**: This command is used to navigate to a directory with white spaces.Instead of using double quotes we can use single quotes then also this command will work.
+
+```
+cd "dir name"
+```