Add basic documentation

Author: sam210723

mimas-v2/sfp/README.md

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+# Small Form-factor Pluggable (SFP) Transceiver Interface
+
+![](https://vksdr.com/bl-content/uploads/pages/8e5fdb2eaf58946d4c3950c0f26ec6d4/sfp.png)
+
+
+## SFP Pmod
+This project uses a custom designed Pmod for connecting SFP transceivers to FPGA development boards. [More info available here](https://vksdr.com/pmod#sfp).
+
+![](https://pbs.twimg.com/media/EuLglpbVcAQRnrV?format=jpg&name=large)
+
+## Layers
+![](docs/phy-layers.png)
+
+ - Physical Medium Dependent (PMD)
+   - SFP transceiver converting between optical signalling and electrical signalling
+ - Physical Medium Attachment (PMA)
+   - NRZ-I Serialisation/Deserialisation
+   - Clock recovery from incoming signal
+ - Physical Coding Sublayer (PCS)
+   - Encoding/Decoding of 4b/5b symbols
+ - Media Independent Interface (MII)
+   - Connection of PCS to MAC
+ - Media Access Controller (MAC)
+   - Framing, [FCS](https://en.wikipedia.org/wiki/Frame_check_sequence) and [IPG](https://en.wikipedia.org/wiki/Interpacket_gap)