Edited .gitignore to filter out build directories and added some general documentation

Signed-off-by: Gabriel Ravier <gabravier@gmail.com>

.gitignore

@@ -1,24 +1,43 @@
-# Misc
+# Exclude obj direcory (object files for Makefile build)
 /obj
+
+# Exclude .dat and .rec files in build directories (avoid Config.dat, 290.rec and others)
 build_en/*.dat
 build_en/*.rec
 build_jp/*.dat
 build_jp/*.rec
+
+# Exclude some other stuff (some of this should probably be removed at some point)
 build.7z
 build.zip
 msvc2003/devilution/orig.asm
 msvc2003/devilution/compare.asm
 
-# Build output
+# Exclude build output on Linux (exclude all files without an extension (executables))
+build_en/*
+!build_en/**/
+!build_en/*.*
+build_jp/*
+!build_jp/**/
+!build_jp/*.*
+
+# Exclude executables in the build folder (and .exe.manifest files)
 build_en/*.exe
 build_en/*.exe.manifest
 build_jp/*.exe
 build_jp/*.exe.manifest
 
-# MSVC trash
+# Exclude the (recommanded) CMake build directory
+build/*
+
+# Exclude MSVC IntelliSense database
 msvc2003/CSE2.ncb
+
+# Exclude MSVC Solution User Options file
 msvc2003/CSE2.suo
+
+# Exclude MSVC build folders
 msvc2003/Debug
 msvc2003/Release
 msvc2003/Debug (Japanese)
-msvc2003/Release (Japanese)
+msvc2003/Release (Japanese)