update README.md with more unit testing examples

Author: lorjala

README.md

@@ -730,7 +730,7 @@ As shown above, tests exist inside `TESTS\testgroup\testcase\` directories. Plea
 
 ## Unit testing
 
-Use the `mbed unittest` command to build and run unit tests, and to generate files for new unit tests.
+Use the `mbed unittest` command to build and run unit tests, or to generate files for new unit tests.
 
 Build and run unit tests with `mbed unittest`. The arguments are:
 
@@ -747,6 +747,46 @@ Build and run unit tests with `mbed unittest`. The arguments are:
 
 Generate files for a new unit test with `mbed unittest --new <FILE>`.
 
+### Building and running unit tests
+
+You can specify to only **build** the unit tests by using the `--skip-run` option:
+
+```
+$ mbed unittest --skip-run
+```
+
+You can specify to only **run** the unit tests by using the `--skip-build` option:
+
+```
+$ mbed unittest --skip-build
+```
+
+If you do not specify any of these, `mbed unittest` will build all available unit tests and run them.
+
+### Running a subset of tests
+
+You can run a **limited set** of unit tests by using the `-r` or `--regex` option. This takes a regular expression, which it compares against the test names. For example to run all cellular unit tests you can specify:
+
+```
+$ mbed unittest -r cellular
+```
+
+### Getting code coverage
+
+You can generate a code coverage report by using the `--coverage` option. For example to create an html report you can specify:
+
+```
+$ mbed unittest --coverage html
+```
+
+### Creating new unit tests
+
+All unit tests are under `mbed-os/UNITTESTS` directory. You can **generate** the necessary files for a unit test by using the `--new` option. For example to create the files for `rtos/Semaphore.cpp` you can specify:
+
+```
+$ mbed unittest --new rtos/Semaphore.cpp
+```
+
 ## Publishing your changes
 
 ### Checking status