Notes-on-gyp.md

+We're trying to rewrite nmake-based build system for Sumatra in [gyp](https://github.com/sumatrapdfreader/sumatrapdf/tree/master/gyp). Gyp is a meta-build system that generates project files (Visual Studio, XCode, makefiles) based on gyp description, which is json-like.
+
+Gyp is not well documented, what I know comes from reverse-engineering by looking at .gyp files from other projects, found with [github code search](https://github.com/search?utf8=%E2%9C%93&q=variables+extension%3Agyp&type=Code&ref=searchresults)
+
+This is a summary of what keys can be used in what context:
+```
+Configuration settings:
+  defines
+  msbuild_toolset : v120_xp, v120
+  msvs_settings*
+
+target_defaults
+  configurations - dict of configurations,that map name to configuration settins
+targets
+  target_name
+  type : executable, shared_library, static_library
+  variables
+  include_dirs
+  sources
+  defines
+  dependencies
+  direct_dependent_settings
+    include_dirs
+  link_settings
+    libraries
+  msvs_disabled_warnings : [4996, 4800]
+  msvs_settings
+    VCCLCompilerTool
+      InlineFunctionExpansion
+      EnableIntrinsicFunctions
+      EnableFunctionLevelLinking
+      FavorSizeOrSpeed : 1 - speed, 
+      OmitFramePointers : true, false
+      Optimization : 0 - none, 2 - max speed 
+      StringPooling : true, false
+      WholeProgramOptimization : true, false
+      RuntimeLibrary : 0 - /MT, 1 - /MTd
+    VCLinkerTool
+      SubSystem : 1 - Console, 2 - Windows
+      EnableCOMDATFolding : 2 - /opt:icf
+      OptimizeReferences : 2 - /opt:ref
+      LinkTimeCodeGeneration : 1 - UseLinkTimeCodeGeneration
+      LinkIncremental : 1 - disable
+```