Add a release notes document for swift-syntax 510

Release Notes/510.md

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+# Swift Syntax 510 Release Notes
+
+## New APIs
+
+- `SyntaxStringInterpolation.appendInterpolation(_: (some SyntaxProtocol)?)`
+  - Descriptions: Allows optional syntax nodes to be used inside string interpolation of syntax nodes. If the node is `nil`, nothing will get added to the string interpolation.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2085
+- `SyntaxCollection.index(at:)`
+  - Description: Returns the index of the n-th element in a `SyntaxCollection`. This computation is in O(n) and `SyntaxCollection` is not subscriptable by an integer.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2014
+
+## API Behavior Changes
+
+## Deprecations
+
+## API-Incompatible Changes
+
+
+## Template
+
+- *Affected API or two word description*
+  - Description: *A 1-2 sentence description of the new/modified API*
+  - Issue: *If an issue exists for this change, a link to the issue*
+  - Pull Request: *Link to the pull request(s) that introduces this change*
+  - Notes: *In case of deprecations or API-incompatible changes, the reason why this change was made and the suggested alternative*
+
+*Insert entries in chronological order, with newer entries at the bottom*