Create PuTTY.gitignore

**Reasons for making this change:**

PuTTYgen is a widely used alternative to OpenSSH under Windows.
This rule allows private keys ignoring.

If this is a new template: 

 - **Link to application or project’s homepage**: https://www.chiark.greenend.org.uk/~sgtatham/putty/

Global/PuTTY.gitignore

+# Private key
+*.ppk