README.md additition

When I run Invoke-DscResourceTest at clean environment, this error occured:

PS C:\> Invoke-DscResourceTest
Unable to find type [Pester.OutputTypes].
At C:\Program Files\WindowsPowerShell\Modules\DscResource.Test\0.16.0\DscResource.Test.psm1:1628 char:9
+         [Pester.OutputTypes]
+         ~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (Pester.OutputTypes:TypeName) [], RuntimeException
    + FullyQualifiedErrorId : TypeNotFound

Problem resolved by running this command before use Invoke-DscResourceTest:

PS C:> New-PesterConfiguration

Run          : Run configuration.
Filter       : Filter configuration
CodeCoverage : CodeCoverage configuration.
TestResult   : TestResult configuration.
Should       : Should configuration.
Debug        : Debug configuration for Pester. ⚠ Use at your own risk!
Output       : Output configuration
TestDrive    : TestDrive configuration.
TestRegistry : TestRegistry configuration.

PS C:\> Invoke-DscResourceTest
cmdlet Invoke-DscResourceTest at command pipeline position 1
Supply values for the following parameters:
ProjectPath:

Please, update documentation.

Update README.md using the reported clean-environment reproduction: run Invoke-DscResourceTest and note the [Pester.OutputTypes] error, then run New-PesterConfiguration before invoking it again. Document the workaround and the expected ProjectPath prompt shown in the issue; no test file is mentioned.