Add link to README.md

Add link to published docs for consistency, context and better UX.

Author: Amanda Butler

blockdevices/BufferedBlockDevice/README.md

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 ## BufferedBlockDevice Example
 
 This BufferedBlockDevice example takes a [HeapBlockDevice](https://os.mbed.com/docs/latest/apis/bufferedblockdevice.html), whose read size is 256 bytes and program size is 512 bytes, and shows how you can read or program this block device with much smaller read/program sizes, using BufferedBlockDevice.
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+To see this example in context, please see its [published documentation](https://os.mbed.com/docs/mbed-os/latest/apis/bufferedblockdevice.html).