Update README to warn about PHPUnit process isolation potential bug

Hello,

As explained in the edited README, I came across a vicous bug using PHPUnit process isolation. When tagging a test with the `@runTestsInSeparateProcesses` annotation, the test's mocks expectation aren't properly checked, resulting in green tests, where they should have been red.

Apparently the Mockyer test listener isn't called in the wrong time/process, and expectations verification doesn't work. You have to explicitely call `Mockery::close` in the test or in the testcase `tearDown`.

That's not really a bug, but this ought to be documented, so others won't spend hours figuring out why tests that shouldn't pass are passing. 

PS: if an english native might proofread my modifications to ensure there's no grammar problems in there, that would be great.