README.md - cache none

Add example of none to turn cache off.

Author: Janne Kiiskilä

README.md

@@ -799,7 +799,7 @@ Here is a list of configuration settings and their defaults:
  * `ARM_PATH`, `GCC_ARM_PATH`, `IAR_PATH` - defines the path to ARM Compiler, GCC ARM and IAR Workbench toolchains. Default: none.
  * `protocol` - defines the default protocol used for importing or cloning of programs and libraries. The possible values are `https`, `http` and `ssh`. Use `ssh` if you have generated and registered SSH keys (Public Key Authentication) with a service such as GitHub, GitLab, Bitbucket and so on. Read more about SSH keys [here](https://help.github.com/articles/generating-an-ssh-key/). Default: `https`.
  * `depth` - defines the *clone* depth for importing or cloning and applies only to *Git* repositories. Note that though this option may improve cloning speed, it may also prevent you from correctly checking out a dependency tree when the reference revision hash is older than the clone depth. Read more about shallow clones [here](https://git-scm.com/docs/git-clone). Default: none.
- * `cache` - defines the local path that stores small copies of the imported or cloned repositories, and mbed CLI uses it to minimize traffic and speed up future imports of the same repositories. Use `on` or `enabled` to turn on caching in the system temp path. Default: none (disabled).
+ * `cache` - defines the local path that stores small copies of the imported or cloned repositories, and mbed CLI uses it to minimize traffic and speed up future imports of the same repositories. Use `on` or `enabled` to turn on caching in the system temp path. Use `none` to turn caching off. Default: none (disabled).
 
 ## Troubleshooting