Add local build instructions (#196)

Author: CorruptComputer

docs/flatpak-maintenance.md

@@ -54,6 +54,19 @@ One thing to note, is that we are using self contained builds in the Flatpak, so
 
 These are required for .NET self contained builds, but are not generated by that tool. If there is an updated version of those it should be as simple as replacing the version numbers in the URL and then just downloading it to check what the sha512sum of the file is locally.
 
+## Building locally
+You'll need the [flatpak-builder](https://docs.flatpak.org/en/latest/flatpak-builder.html) tools installed. 
+
+In the Flatpak package repo you have checked out you can run this:
+```
+flatpak-builder ./build org.unitystation.StationHub.yaml --install --user --force-clean
+```
+
+Once that completes it'll be installed it locally for your user, and you can launch it with:
+```
+flatpak run org.unitystation.StationHub --user
+```
+
 ## Other info
 ### Permissions
 Flatpaks have a permissions system, in `org.unitystation.StationHub.yaml` we have these permissions setup currently:
@@ -87,5 +100,19 @@ The meta information for the app is specified in the metainfo XML file here: htt
 This will be verified as valid before the build is accepted, and the guidelines for this are very strict.
 For example, the releases must be specified in newest to oldest order or the build is rejected.
 
+The metainfo can be validated locally by using the following:
+```
+appstream-util validate org.unitystation.StationHub.metainfo.xml
+```
+
+If you get errors like these, they can be ignored.
+```
+org.unitystation.StationHub.metainfo.xml: FAILED:
+• attribute-invalid     : <screenshot> width too large [https://unitystationfile.b-cdn.net/screenshots/unitystation-monke.png] maximum is 1600px
+• attribute-invalid     : <screenshot> height too large [https://unitystationfile.b-cdn.net/screenshots/unitystation-monke.png] maximum is 900px
+• attribute-invalid     : <screenshot> width too large [https://unitystationfile.b-cdn.net/screenshots/cargo-ui.jpg] maximum is 1600px
+• attribute-invalid     : <screenshot> height too large [https://unitystationfile.b-cdn.net/screenshots/cargo-ui.jpg] maximum is 900px
+```
+
 ### Desktop entry
 The desktop entry is specified in this file: https://github.com/unitystation/stationhub/blob/develop/UnitystationLauncher/Assets/org.unitystation.StationHub.desktop