added a simple register example

examples/register/README.md

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+# Register
+
+## Description
+
+This program illustrates as simply as possible how to use registers in P4 and
+read / write the register state using the bmv2 runtime CLI.
+
+This will probably change in the future but as of now the data plane is not
+doing anything (so don't try to send packets). However you can take a look at
+the P4 source code and at the read_register and write_register scripts. You can
+also run the following demo:
+
+### Running the demo
+
+To run the demo:
+- start the switch with `./run_switch.sh`.
+- write a register cell with `write_register.sh`. For example:
+  `./write_register.sh 123 88` to set `my_register[123]` to 88.
+- read a register cell with `read_register.sh`. For example:
+  `./read_register.sh 123` to read `my_register[123]`.