readme: improvements to installation requirements
readme: improvements to installation requirements

sergeche/libxmljs,libxmljs/libxmljs,Visya/libxmljs,Konsus/libxml,apeace/libxmljs,hildjj/libxmljs,Konsus/libxml,rchipka/libxmljs,Konsus/libxml,Visya/libxmljs,libxmljs/libxmljs,isaacs/libxmljs,libxmljs/libxmljs,apeace/libxmljs,CodeFridge/libxmljs_mem,improvisio-software/libxmljs,hildjj/libxmljs,rc0x03/libxmljs,gagern/libxmljs,polotek/libxmljs,rc0x03/libxmljs,gagern/libxmljs,polotek/libxmljs,isaacs/libxmljs,CodeFridge/libxmljs_mem,improvisio-software/libxmljs,Visya/libxmljs,gagern/libxmljs,hildjj/libxmljs,rc0x03/libxmljs,sergeche/libxmljs,improvisio-software/libxmljs,isaacs/libxmljs,rchipka/libxmljs,isaacs/libxmljs,polotek/libxmljs,CodeFridge/libxmljs_mem,rchipka/libxmljs,sergeche/libxmljs