Add SDL dev docs

doc/developer.md

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+# Developer Docs for SDL
+
+Random stuff that is useful for devs.
+
+# Checking return values from litelib
+
+There are 3 functions written in Rust that live in lib/src/lib.rs :
+
+  * `litelib_initialize_new`
+    * create a new client/connection and brand new wallet
+  * `litelib_initialize_new_from_phrase`
+    * create a new client/connection from a seedphrase (restoring from seedphrase)
+  * `litelib_initialize_existing`
+    * create a new client/connection with an already existing wallet
+
+The Rust code calls it a "LightClient" while the C++ of SDL calls it a "Connection".
+
+When `litelib_initialize_existing` or `litelib_initialize_new_from_phrase` return successfully, they return the string "OK" (which is not JSON).
+
+When `litelib_initialize_new` returns successfully it returns JSON that looks like :
+
+```
+{"seed":"seed","birthday":birthday}
+```
+
+where "seed" is a 24 word seed and birthday is an integer block height.
+
+So when calling these 3 functions, which looks almost the same in the calling code, the code which checks if they worked will be different, depending on what each returns on success.
+
+When checking the return value of `litelib_initialize_existing` or `litelib_initialize_new_from_phrase` it should look like :
+
+```
+QString reply = "";
+try {
+    char* resp = litelib_initialize_new_from_phrase(...);
+    reply = litelib_process_response(resp);
+} catch {
+    ...
+}
+if (reply.isEmpty())) {
+    // litelib_initialize_new_from_phrase failed
+    ...
+}
+```
+
+When checking the return value of `litelib_initialize_new`  it should look like :
+
+```
+QString reply = "";
+try {
+    char* resp = litelib_initialize_new(...);
+    reply = litelib_process_response(resp);
+} catch {
+    ...
+}
+if (reply.toUpper().trimmed() != "OK") {
+    // litelib_initialize_new failed
+    ...
+}
+```