Add README's for top level folders to help folks navigate the project (

#473)

* Add README's for top level folders to help folk navigate the project

* Small corrections to spelling and links

extensions/README.md

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+# Extensions
+
+This folder contains all the extensions for individual service providers
+and cloud platforms.
+
+ - **auth** contains authorization libraries used by service providers to
+   authorize users.
+
+ - **cloud** contains cloud platform specific extensions that allow DTP to
+   be run on various public clouds.
+
+ - **data-transfer** contains the service provider specific adapters
+   that translate a service's API into one or more 
+   [DTP data models](../portability-types-transfer/src/main/java/org/datatransferproject/types/transfer/models).
+
+ - **transport** contains implementations of various transport layers that
+ can be used to communicate with the DTP platform. 

libraries/README.md

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+# Libraries
+
+This folder contains shared logic/libraries used by core DTP code.
+
+ - **config** contains utilities for configuring DTP via flags and config files. 
+
+ - **security** contains cryptographic helper libraries.
+
+ - **transfer** contains helper utilities for facilitating the transfer of data.
+

portability-api/README.md

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+# portability-api
+
+This folder contains the binary for the API server used by DTP.
+
+DTP has two servers: the API, and the Worker.  The API server exposes DTP's
+API (hence the name).  This allows clients to submit jobs, and check job
+statuses.  The [Worker](../portability-transfer) does the actual transfer
+of the data.
+

portability-spi-api/README.md

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+# portability-spi-api
+
+This folder contains the Service Provider Interface (SPI) for DTP's
+API server.
+
+Service providers looking to be included in DTP will need to implement
+[AuthDataGenerator](src/main/java/org/datatransferproject/spi/api/auth/AuthDataGenerator.java)
+and [AuthServiceExtension](src/main/java/org/datatransferproject/spi/api/auth/extension/AuthServiceExtension.java).
+

portability-spi-cloud/README.md

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+# portability-spi-cloud
+
+This folder contains the Service Provider Interface (SPI) for DTP's
+Cloud abstractions.
+
+DTP is designed to be runnable from any cloud infrastructure, or locally.
+
+This folder contains the SPIs needed from any cloud implementation.
+
+[Current cloud implementations](../extensions/cloud).
+
+If you want to add support for a new cloud platform, simply add
+implementations of these interfaces to a new project in the above folder. 

portability-spi-transfer/README.md

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+# portability-spi-transfer
+
+This folder contains the Service Provider Interface (SPI) for DTP's
+transfer abstractions.
+
+If you are [adding a new service provider integration](../Documentation/Integration.md) 
+these are the interfaces needed.
+
+See our [current integrations](../extensions/data-transfer) for example of implementation of
+these interfaces.

portability-transfer/README.md

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+# portability-transfer
+
+This folder contains the binary for the Worker server used by DTP.
+
+DTP has two servers: the API, and the Worker.  The [API server](../portability-api)
+exposes DTP's API (hence the name).  This allows clients to submit jobs,
+and check job statuses.  The Worker does the actual transfer
+of the data.

portability-types-transfer/README.md

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+# portability-types-transfer
+
+This folder contains the common data models that are used through out the DTP.
+
+ - [auth](src/main/java/org/datatransferproject/types/transfer/auth) -
+   common data structures for passing auth data around the DTP system 
+ - [models](src/main/java/org/datatransferproject/types/transfer/models) -
+   common data structures for encoding the data to be transferred
+ - [retry](src/main/java/org/datatransferproject/types/transfer/retry) -
+   common data structures for encoding the retry behavior for a server when
+   there is an error.