Update README.md

README.md

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 This is a library to parse ELF's exception handling information out of its binary form and into it's "parse tree" form.
 
 The API is documented in the include directory.  The source tree is complicated and understanding it should not be necessary.
@@ -10,3 +9,4 @@ Notes:
 1. Compilation requires C++11 or later.
 1. Additional documentation will be provided in later versions 
 1. API is incomplete and untested in some areas.  Future versions will improve stability.
+1. Use git clone --recursive to pull down required submodules