Refactor build resources

Move DUB files into /bld/d and add more instructions to README.

.gitignore

@@ -11,8 +11,6 @@
 !build.sh
 !CHANGELOG
 !configure.ac
-!dub.sdl
-!dub.selections.json
 !extension_api.json
 !LICENSE
 !Makefile.am
@@ -21,6 +19,21 @@
 
 !/bin/main.gdextension
 
+# /bld
+!/bld/
+/bld/*
+
+# /bld/src
+!/bld/src/
+/bld/src/*
+
+# /bld/src/d
+!/bld/src/d/
+/bld/src/d/*
+
+!/bld/src/d/dub.sdl
+!/bld/src/d/dub.selections.json
+
 # /docs
 !/docs/
 /docs/*
@@ -64,19 +77,17 @@
 !/src/init
 /src/init/*
 
-!/src/init/bind.d
-
 # /src/init/cpp
-!/src/init/cpp
+!/src/init/cpp/
 /src/init/cpp/*
 
 !/src/init/cpp/bind.cpp
 
 # /src/init/d
-!/src/init/d
+!/src/init/d/
 /src/init/d/*
 
-/src/init/d/bind.d
+!/src/init/d/bind.d
 
 !/src/init/d