Make PR from #324

Make new PR from old PR I'm closing to avoid merge conflicts.

Author: Amanda Butler

docs/reference/contributing/storage/storage.md

@@ -1,3 +1,32 @@
 <h2 id="contributing-storage">Storage</h2>
 
-The two storage APIs are BlockDevice and FileSystem. These APIs provide filesystem and file operations on a block device, as well as an interface for access to block-based storage.
+Storage support is split between file systems and their underlying block device support. The <a href="/docs/v5.6/reference/storage-overview.html" target="_blank">storage API page</a> has more information on existing APIs in Mbed OS for both interfaces.
+
+#### Block Device
+
+Adding a block device implementation is required for backing filesystems on new hardware. You can extend the <a href="https://os.mbed.com/docs/v5.6/mbed-os-api-doxy/class_heap_block_device.html" target="_blank">BlockDevice</a> class to provide support for unsupported storage. 
+
+If you want to port a new file system to Mbed OS on existing storage options you can skip to the following section.
+
+#### File systems
+
+To implement a new file system in Mbed OS, an implementor needs to provide the abstract functions in the file system interface. The <a href="https://os.mbed.com/docs/v5.6/mbed-os-api-doxy/class_f_a_t_file_system.html" target="_blank">FAT file system</a> provides an excellent example.
+
+A minimal file system needs to provide the following functions:
+
+- `file_open`.
+- `file_close`.
+- `file_read`.
+- `file_write`.
+- `file_seek`.
+
+Here is the full API that a file system may implement:
+
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/v5.7/mbed-os-api-doxy/classmbed_1_1_file_system.html)
+
+File systems must be backed by a block device in Mbed OS. If you are using supported hardware, then you can use the Mbed OS block device classes. Otherwise, view the block device porting section earlier in this guide.
+
+#### Related content
+
+- <a href="https://os.mbed.com/docs/v5.6/mbed-os-api-doxy/class_heap_block_device.html" target="_blank">BlockDevice</a>.
+- <a href="https://os.mbed.com/docs/v5.6/mbed-os-api-doxy/class_f_a_t_file_system.html" target="_blank">FAT file system</a>.