Update VERSION in Makefile and add documentation for string_view_t #252

Job	Run time
windows (Win32, standard)	1m 32s
ubuntu (g++, standard)	1m 5s
windows (Win32, PUGIXML_WCHAR_MODE)	2m 14s
windows (Win32, PUGIXML_COMPACT)	1m 51s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1m 5s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	2m 3s
ubuntu (g++, PUGIXML_COMPACT)	1m 7s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	1m 1s
ubuntu (g++, PUGIXML_STRING_VIEW)	1m 7s
windows (Win32, PUGIXML_STRING_VIEW)	2m 26s
ubuntu (clang++, standard)	51s
windows (x64, standard)	1m 38s
windows (x64, PUGIXML_WCHAR_MODE)	2m 39s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	53s
ubuntu (clang++, PUGIXML_COMPACT)	54s
windows (x64, PUGIXML_COMPACT)	1m 49s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	58s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 14s
ubuntu (clang++, PUGIXML_STRING_VIEW)	56s
windows (x64, PUGIXML_STRING_VIEW)	1m 46s
macos (clang++, standard)	1m 11s
macos (clang++, PUGIXML_WCHAR_MODE)	47s
macos (clang++, PUGIXML_COMPACT)	1m 4s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	51s
macos (clang++, PUGIXML_STRING_VIEW)	1m 1s
	34m 3s

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