Add relnotes to repo

doc/relnotes.md

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+# SilentDragonLite v1.5.3 ""
+
+    * Update silentdragonlite-cli dependency
+        * This makes syncing new wallets drastically faster
+    * Add backend server lite.hushpool.is
+    * Fix various bugs in the New Wallet Wizard
+    * Ignore leading/trailing spaces in wallet birthday when importing a seed
+    * Show QT Version in About screen
+    * Removed confusing text about mining
+    * Update translations for existing languages
+
+# SilentDragonLite v1.5.2 "Zany Zulu"
+
+    * Fixes connection problems for older versions
+    * Updated one of the community servers to new domain: lite.hush.land
+    * Fix bug where trailing slash on server name crashes SDL
+    * Improved error-handling of invalid data at libsodium layer
+    * Avoid showing corrupted ciphertext in GUI
+