document DLL updates

RELEASING.md

@@ -1,4 +1,9 @@
 # Making a release
+## Check for DLL updates
+The draft release notes probably already mention them, but check the [mxe releases](https://github.com/UltraStar-Deluxe/mxe/releases) for any pre-release versions.
+If there are any pre-releases present, you probably need to check [UPDATING-DLLS.md](UPDATING-DLLS.md).
+
+## Releasing USDX
 1. Find the contents of `VERSION` (strip the `+dev`) throughout the code.
     This should result in six places:
     * [VERSION](VERSION)
@@ -20,4 +25,4 @@
     Don't forget to also create a PR for this release in
     https://github.com/UltraStar-Deluxe/ultrastar-deluxe.github.io
 7. Create a PR in [the FlatHub repository](https://github.com/flathub/eu.usdx.UltraStarDeluxe) that updates the tag and commit values
-    See this PR for an example: https://github.com/flathub/eu.usdx.UltraStarDeluxe/pull/7/files
+    See this PR for an example: https://github.com/flathub/eu.usdx.UltraStarDeluxe/pull/7/files

UPDATING-DLLS.md

@@ -0,0 +1,16 @@
+# Updating DLLs
+The Windows DLLs are stored as releases in the [mxe](https://github.com/UltraStar-Deluxe/mxe) repository.
+
+If you need to update them:
+1. Open a PR in the [mxe](https://github.com/UltraStar-Deluxe/mxe) repository
+2. Open a PR in the USDX repository that updates the commit id in [dldlls.py](dldlls.py)
+3. Once everything works, merge the PR in the mxe repository
+4. Update the PR in the USDX repository to point to the commit id of the merge commit from step 3
+5. Merge the PR in the USDX repository and make a pre-release in the mxe repository. Pre-releases have `-rc1` added to the name of both the release and the zip asset!
+
+## Release updated DLLs
+When releasing a new version of USDX that uses new DLLs for the first time, edit the pre-release:
+* change the name (name it the same the USDX version you're about to release)
+* rename the zip asset
+* unmark it as pre-release
+* set as latest release