Add info on skipped tests and how to fix them (#547)

* Add info on skipped tests and how to fix them

* Detail PHPAR env vars in contribution docs.

* Clarifies that brew commands are for macOS

Author: louismrose

CONTRIBUTING.md

@@ -26,3 +26,18 @@ Or run a single test file by specifying its path:
 ```sh
 vendor/bin/phpunit test/InflectorTest.php
 ```
+
+#### Skipped Tests ####
+
+You might notice that some tests are marked as skipped. To obtain more information about skipped
+tests, pass the `--verbose` flag to PHPUnit:
+
+```sh
+vendor/bin/phpunit --verbose
+```
+
+Some common steps for fixing skipped tests are to:
+
+* Install `memcached` and the PHP memcached extension (e.g., `brew install php56-memcache memcached` on macOS)
+* Install the PDO drivers for PostgreSQL (e.g., `brew install php56-pdo-pgsql` on macOS)
+* Create a MySQL database and a PostgreSQL database. You can either create these such that they are available at the default locations of `mysql://test:[email protected]/test` and `pgsql://test:[email protected]/test` respectively. Alternatively, you can set the `PHPAR_MYSQL` and `PHPAR_PGSQL` environment variables to specify a different location for the MySQL and PostgreSQL databases.