Update documentation - Test/LinuxMain.swift was deleted (#141)

* Update documentation - Test/LinuxMain.swift was deleted

Test/LinuxMain.swift was deleted in commit 4a33a0f - tag: 2.0.0-beta.2

* Update sentence about tests.

Author: jccampagne

CONTRIBUTING.md

@@ -18,8 +18,7 @@ Be careful to observe the script's output, it may have errors or ask you to do a
 
 Once in Xcode, select the `FluentPostgreSQL-Package` scheme and use `CMD+U` to run the tests.
 
-When adding new tests (please do 😁), don't forget to add the method name to the `allTests` array. 
-If you add a new `XCTestCase` subclass, make sure to add it to the `Tests/LinuxMain.swift` file.
+Please add tests to cover bug fixes and new features.
 
 If you are fixing a single GitHub issue in particular, you can add a test named `testGH<issue number>` to ensure
 that your fix is working. This will also help prevent regression.