Update README.md

[jonp200]

While using the Go client (github.com/Shopify/toxiproxy/client), I found that after creating a proxy and adding toxics via proxy.AddToxic(...), the toxic does not take effect unless I explicitly call proxy.Save() afterward.

This is not currently mentioned in the documentation or examples, and it caused some confusion when toxics weren’t being applied despite no errors during creation.

Example:

proxy, _ := client.CreateProxy("my_proxy", "localhost:5000", "localhost:6000")
proxy.AddToxic("latency_downstream", "latency", "downstream", 1.0, client.Attributes{
    "latency": 100,
})
proxy.Save() // Without this, the toxic is not active

Suggestions:

• Mention this requirement in the README and/or inline GoDoc.
• Possibly make AddToxic call Save() internally, or clarify the design choice if not.

[jonp200]

I have signed the CLA!

[abecevello]

Looks great! Thanks for the contribution.