[otbn] Initial README
Add a basic README to the `hw/ip/otbn` directory. This README holds
developer-focused "tips and tricks," but is not intended to have any
user-facing documentation in it (we do have the specification and
related documents for that).
Signed-off-by: Philipp Wagner <phw@lowrisc.org>
diff --git a/hw/ip/otbn/README.md b/hw/ip/otbn/README.md
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+# OpenTitan Big Number Accelerator (OTBN)
+
+This directory contains the implementation of the OpenTitan Big Number
+Accelerator (OTBN). OTBN is a coprocessor for asymmetric cryptographic
+operations like RSA or Elliptic Curve Cryptography (ECC).
+
+See https://docs.opentitan.org/hw/ip/otbn/doc/index.html for documentation on
+the current version of OTBN; documentation matching the code in this directory
+can be found in the `doc` directory.
+
+OTBN is currently in early development. Please ask questions and report issues
+through the [GitHub issue tracker](https://github.com/lowRISC/opentitan/issues).
+
+## Develop OTBN
+
+## Build the RTL implementation
+
+To build the RTL implementation of OTBN in a simulation, run `fusesoc` without
+passing the `OTBN_MODEL` flag. For example, the Verilator simulation can be
+compiled as follows.
+
+```sh
+fusesoc --cores-root=. run --target=sim --setup --build lowrisc:systems:top_earlgrey_verilator
+```
