Add readme sample for web root path variable

[feedmeapples]

What was changed

Adds readme sample of TEMPORAL_WEB_ROOT_PATH value

Why?

Web root path works only if passed in a format with forward slashes on both sides. This value is passed to koa-router, webpack and used in http.js.

Another option would be to do custom processing and modify the value so it fits the format.
The below regex can be used to trim forward slashes on both ends of the path. Then the resulting value can be passed to koa-router, webpack and http.js according to their expected format (adding forward slashes if needed). Having slashes on both sides works for all places

"/test/root/path/".replace(/(^\/|\/$)/, '')

Checklist

1. Closes issue:

2. How was this tested:

Ran with TEMPORAL_WEB_ROOT_PATH set to "/test/"

3. Any docs updates needed?

No

[swyxio]

examples always a good idea!