added READMEs for the bridges

src/Symfony/Bridge/Doctrine/README.md

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+Doctrine Bridge
+===============
+
+Provides integration for Doctrine with various Symfony2 components.
+
+Resources
+---------
+
+Unit tests:
+
+https://github.com/symfony/symfony/tree/master/tests/Symfony/Tests/Bridge/Doctrine

src/Symfony/Bridge/Monolog/README.md

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+Monolog Bridge
+==============
+
+Provides integration for Monolog with various Symfony2 components.
+
+Resources
+---------
+
+Unit tests:
+
+https://github.com/symfony/symfony/tree/master/tests/Symfony/Tests/Bridge/Monolog

src/Symfony/Bridge/Twig/README.md

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+Twig Bridge
+===========
+
+Provides integration for Twig with various Symfony2 components.
+
+Resources
+---------
+
+Unit tests:
+
+https://github.com/symfony/symfony/tree/master/tests/Symfony/Tests/Bridge/Twig