Created Allow running indoors (markdown)

Allow-running-indoors.md

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+Credit to TheXaman for posting this in the Simple Modifications Pokecommunity thread.
+
+This tutorial will allow the player to run on any map.
+
+Edit [src/bike.c](../blob/master/src/bike.c):
+```diff
+ bool32 IsRunningDisallowed(u8 metatile)
+ {
+-    if (!(gMapHeader.flags & MAP_ALLOW_RUNNING) || IsRunningDisallowedByMetatile(metatile) == TRUE)
++    if (IsRunningDisallowedByMetatile(metatile) == TRUE)
+         return TRUE;
+     else
+         return FALSE;
+ }
+```
+And that's it!