Updated README

README.md

 This is an example to export ECL barrel temperature/humidity to EPICS
 using pcaspy python module.
+
+## Usage
+
+1. Configure iptables according to instructions in 00_iptables.
+2. Set up the environment: `source setenv.sh` (only supported in bash shell).
+3. Install externals (specifically, [pcaspy](https://pypi.org/project/pcaspy/) module) using `01_get_externals.sh`.
+4. Start EPICS CA server: `./02_epics_ca.py`.