chore: add readme for site-specific-feature (#799)

Author: dbajpeyi

docs/site-specific-feature.md

@@ -0,0 +1,53 @@
+## `src/site-specific-feature.js`
+
+The [site-specific-feature.js](https://github.com/duckduckgo/duckduckgo-autofill/blob/main/src/site-specific-feature.js) module provides APIs to find elements on a document, based on settings specified in the `siteSpecificFixes` feature in [privacy-configuration](https://github.com/duckduckgo/privacy-configuration). These are used during scanning, input matching or form creation to apply site specific fixes, which is useful when the default algorithm fails to identify elements algorithmically. It extends the [ConfigFeature](https://github.com/duckduckgo/content-scope-scripts/blob/main/injected/src/config-feature.js) class from content-scope-scripts, which provides APIs to read remote config settings.
+
+Currently, it supports following type of fixes:
+
+### Input type
+Force an input element on the page to be a specific type.
+
+- `inputTypeSettings`: This is an array of objects that contain the following properties:
+- `selector`: A string that matches the input type.
+- `type`: The type of input to apply - `login`, `signup`, or `hybrid`.
+
+Example setting in remote config:
+```json
+{
+  "inputTypeSettings": [
+    {
+      "selector": "input[type='email']",
+      "type": "identities.emailAddress"
+    }
+  ]
+}
+```
+
+
+### Form boundary
+Force the form boundary to be a specific element on the page.
+- `formBoundarySelector`: A string that matches the form on the page.
+
+Example setting in remote config:
+```json
+{
+  "formBoundarySelector": "form[name='login']"
+}
+```
+
+### Form type
+Force a form on a page to be a specific type.
+- `selector`: A string that matches the form we want to fix.
+- `type`: The type of form to apply, this matches the [SupportedTypes](https://github.com/duckduckgo/duckduckgo-autofill/blob/main/src/Form/matching.js#L755) we have in the code.
+
+Example setting in remote config:
+```json
+{
+  "formTypeSettings": [
+    {
+      "selector": "form[name='login']",
+      "type": "identities.emailAddress"
+    }
+  ]
+}
+```