[Docs] Add docs for wasm-merge export handling flags (#5725)

Author: kripken

README.md

@@ -574,6 +574,11 @@ file the calls between the modules become just normal calls inside a module,
 which allows them to be inlined, dead code eliminated, and so forth, potentially
 improving speed and size.
 
+`wasm-merge` operates on normal wasm files. It differs from `wasm-ld` in that
+respect, as `wasm-ld` operates on wasm *object* files. `wasm-merge` can help
+in multi-toolchain situations where at least one of the toolchains does not use
+wasm object files.
+
 For example, imagine we have these two wasm files:
 
 ```wat
@@ -709,6 +714,33 @@ merged.exports.main();
 We still need to fetch and compile the merged wasm, and to provide it the JS
 import, but the work to connect two wasm modules is not needed any more.
 
+#### Handling exports
+
+By default `wasm-merge` errors if there are overlapping export names. That is,
+`wasm-merge` will automatically handle overlapping function names and so forth,
+because those are not externally visible (the code still behaves the same), but
+if we renamed exports then the outside would need to be modified to expect the
+new export names, and so we error instead on such name conflicts.
+
+If you do want exports to be renamed, run `wasm-merge` with
+`--rename-export-conflicts`. Later exports will have a suffix appended to them
+to ensure they do not overlap with previous exports. The suffixes are
+deterministic, so once you see what they are you can call them from the outside.
+
+Another option is to use `--skip-export-conflicts` which will simply skip later
+exports that have conflicting names. For example, this can be useful in the
+case where the first module is the only one that interacts with the outside and
+the later modules just interact with the first module.
+
+#### Features
+
+`wasm-merge` uses the multi-memory and multi-table features. That is, if
+multiple input modules each have a memory then the output wasm will have several
+memories, and will depend on the multi-memory feature, which means that older
+wasm VMs might not be able to run the wasm. (As a workaround for such older VMs
+you can run `wasm-opt --multi-memory-lowering` to lower multiple memories into a
+single one.)
+
 ## Testing
 
 ```