Update README with usage

README.md

 # docker-ocserv
-Docker OpenConnect VPN Server
+
+docker-ocserv is an OpenConnect VPN Server boxed in a Docker image built by [Tommy Lau](mailto:tommy@gen-new.com).
+
+## What is OpenConnect Server?
+
+[OpenConnect server (ocserv)](http://www.infradead.org/ocserv/) is an SSL VPN server. It implements the OpenConnect SSL VPN protocol, and has also (currently experimental) compatibility with clients using the [AnyConnect SSL VPN](http://www.cisco.com/c/en/us/support/security/anyconnect-vpn-client/tsd-products-support-series-home.html) protocol.
+
+## How to use this image
+
+Get the docker image by running the following commands:
+
+```bash
+docker pull tommylau/ocserv
+```
+
+Start an ocserv instance:
+
+```bash
+docker run --name ocserv --privileged -p 443:443 -d tommylau/ocserv
+```
+
+This will start an instance with the a test user named `test` and password is also `test`.
+
+### Environment Variables
+
+All the variables to this image is optional, which means you don't have to type in any environment variables, and you can have a OpenConnect Server out of the box! However, if you like to config the ocserv the way you like it, here's what you wanna know.
+
+`CA_CN`, this is the common name used to generate the CA(Certificate Authority).
+
+`CA_ORG`, this is the organization name used to generate the CA.
+
+`CA_DAYS`, this is the expiration days used to generate the CA.
+
+`SRV_CN`, this is the common name used to generate the server certification.
+
+`SRV_ORG`, this is the organization name used to generate the server certification.
+
+`SRV_DAYS`, this is the expiration days used to generate the server certification.
+
+`NO_TEST_USER`, while this variable is set to not empty, the `test` user will not be created. You have to create your own user with password. The default value is to create `test` user with password `test`.
+
+The default values of the above environment variables:
+
+|   Variable   |     Default     |
+| : -------- : | : ----------- : |
+|  **CA_CN**   |      VPN CA     |
+|  **CA_ORG**  |     Big Corp    |
+| **CA_DAYS**  |       9999      |
+|  **SRV_CN**  | www.example.com |
+| **SRV_ORG**  |    My Company   |
+| **SRV_DAYS** |       9999      |
+
+### Running examples
+
+Start an instance out of the box with username `test` and password `test`
+
+```bash
+docker run --name ocserv --privileged -p 443:443 -d tommylau/ocserv
+```
+
+Start an instance with server name `my.test.com`, `My Test` and `365` days
+
+```bash
+docker run --name ocserv --privileged -p 443:443 -e SRV_CN=my.test.com -e SRV_ORG="My Test" -e SRV_DAYS=365 -d tommylau/ocserv
+```
+
+Start an instance with CA name `My CA`, `My Corp` and `3650` days
+
+```bash
+docker run --name ocserv --privileged -p 443:443 -e CA_CN="My CA" -e CA_ORG="My Corp" -e CA_DAYS=3650 -d tommylau/ocserv
+```
+
+A totally customized instance with both CA and server certification
+
+```bash
+docker run --name ocserv --privileged -p 443:443 -e CA_CN="My CA" -e CA_ORG="My Corp" -e CA_DAYS=3650 -e SRV_CN=my.test.com -e SRV_ORG="My Test" -e SRV_DAYS=365 -d tommylau/ocserv
+```
+
+Start an instance as above but without test user
+
+```bash
+docker run --name ocserv --privileged -p 443:443 -e CA_CN="My CA" -e CA_ORG="My Corp" -e CA_DAYS=3650 -e SRV_CN=my.test.com -e SRV_ORG="My Test" -e SRV_DAYS=365 -e NO_TEST_USER=1 -v /some/path/to/ocpasswd:/etc/ocserv/ocpasswd -d tommylau/ocserv
+```
+
+**WARNING:** The ocserv requires the ocpasswd file to start, if `NO_TEST_USER=1` is provided, there will be no ocpasswd created, which will stop the container immediately after start it. You must specific a ocpasswd file pointed to `/etc/ocserv/ocpasswd` by using the volume argument `-v` by docker as demonstrated above.
+
+### User operations
+
+All the users opertaions happened while the container is running. If you used a different container name other than `ocserv`, then you have to change the container name accordingly.
+
+#### Add user
+
+If say, you want to create a user named `tommy`, type the following command
+
+```bash
+Enter password:
+Re-enter password:
+```
+
+When prompt for password, type the password twice, then you will have the user with the password you want.
+
+#### Delete user
+
+Delete user is similar to add user, just add another argument `-d` to the command line
+
+```bash
+docker exec -ti ocserv ocpasswd -c /etc/ocserv/ocpasswd -d test
+```
+
+The above command will delete the default user `test`, if you start the instance without using environment variable `NO_TEST_USER`.
+
+#### Change password
+
+Change password is exactly the same command as add user, please refer to the command mentioned above.