Update README.md with GAM Secure Signals how-to (#167)

Author: MO-Thibault

README.md

@@ -20,6 +20,7 @@ JavaScript SDK for integrating with an [Optable Data Connectivity Node (DCN)](ht
   - [Targeting key values](#targeting-key-values)
   - [Targeting key values from local cache](#targeting-key-values-from-local-cache)
   - [Witnessing ad events](#witnessing-ad-events)
+  - [Passing Secure Signals to GAM](#gam-secure-signals)
 - [Integrating Prebid](#integrating-prebid)
   - [Seller Defined Audiences](#seller-defined-audiences)
   - [Custom key values](#custom-key-values)
@@ -418,6 +419,32 @@ Note that you can call `installGPTEventListeners()` as many times as you like on
 
 A working example of both targeting and event witnessing is available in the demo pages.
 
+### GAM Secure Signals
+
+The Optable Web SDK provides a method `installGPTSecureSignals` to pass user-defined signals to Google Ad Manager (GAM) [Secure Signals](https://support.google.com/admanager/answer/10488752). The method supports an array of objects, each representing a unique signal to pass to GAM, for example:
+
+```html
+<!-- Optable SDK async load: -->
+<script async src="https://cdn.optable.co/web-sdk/latest/sdk.js"></script>
+<script>
+  const mySecureSignals = [];
+  mySecureSignals.push({
+    // Name of the provider
+    provider: "uidapi.com",
+    // ID to use in the Secure Signal
+    value: "uid2_token_goes_here",
+  });
+  window.optable = window.optable || { cmd: [] };
+  optable.cmd.push(function () {
+    optable.instance.installGPTSecureSignals(mySecureSignals);
+  });
+</script>
+```
+
+Please refer to the list of approved Secure Signal [providers](https://support.google.com/admanager/answer/14750072). Please refer to the provider's integration documentation for the exact provider name and value to pass as a signal.
+
+You can verify the signal was correctly passed to GAM by searching for its value cached in `localStorage` under the key `_GESPSK-<provider_name>`.
+
 ## Integrating Prebid
 
 The Optable Web SDK can fetch targeting data from a DCN and prepare an audience taxonomy object similar to the one described in [the prebid.js first party data documentation](https://docs.prebid.org/features/firstPartyData.html#segments-and-taxonomy). The `prebidORTB2FromCache()` function returns the object from the targeting data stored by `targeting()` API calls in `LocalStorage`.