Add mbed CLI configuration section to README

Author: screamerbg

README.md

@@ -34,6 +34,7 @@ This document covers the installation and usage of *mbed CLI*.
   2. [Change the test action](#change-the-test-action)
   3. [Limiting the test scope](#limiting-the-test-scope)
   4. [Test directory structure](#test-directory-structure)
+1. [mbed CLI configuration](#mbed-cli-configuration)
 1. [Known limitations](#known-limitations)
 
 ## Installation
@@ -689,6 +690,31 @@ As shown above, tests exist inside ```TESTS\testgroup\testcase\``` directories.
 
 <span class="notes">**Note:** This feature does not work in applications that contain a  ```main``` function that is outside of a `TESTS` directory.</span>
 
+## mbed CLI configuration
+
+Many options in *mbed CLI* can be streamlined with global and local configuration.
+
+The *mbed CLI* configuration syntax is:
+```
+mbed config [--global] <var> [value] [--unset]
+```
+
+The **global** configuration (via `--global` option) defines the default behavior of *mbed CLI* across programs unless overridden by *local* settings.
+
+The **local** configuration (without `--global`) is per mbed program and allows overriding of global or default *mbed CLI* settings within the scope of a program or library and its dependencies.
+
+If **no value** is specified then *mbed CLI* will print the currently set value for this settings from either local or global scope.
+
+The `--unset` option allows removing of a setting.
+
+Here is a list of currently implemented configuration settings:
+ * `target` - Define default target for `compile`, `test` and `export`, and is an alias to `mbed target`. Default: none.
+ * `toolchain` - Defines default toolchain for `compile` and `test` can be set through `mbed toolchain`. Default: none.
+ * `ARM_PATH`, `GCC_ARM_PATH`, `IAR_PATH` - defines default path to ARM Compiler, GCC ARM and IAR Workbench toolchains. Default: none.
+ * `protocol` - Defines the default protocol used for importing/cloning of programs and libraries. Possible values are `https`, `http` and `ssh`. Default: `https`.
+ * `depth` - Defines *clone* depth for importing/cloning and applies only to *Git* repositories. Note that while this option may improve cloning speed, it may also prevent you from correctly checking out a dependency tree when the reference revision hash older than the clone depth. Google `shallow clone` if you'd like to know more about using shallow clones. Default: none.
+ * `cache` (EXPERIMENTAL) - Defines local path that will be used to store minimalistic copies of the imported/cloned repositories and attempt to use them to minimize traffic and speed up future importing. Note that this is an experimental feature and should be used for stable and production-ready repositories only. Default: none (disabled).
+
 ## Known limitations
 
 <span class="warnings">**Warning**: At this point, *mbed CLI* is alpha quality and very much in development. Breakages are fully expected. Please open issues on this repository for any problems that you find with *mbed CLI*.</span>