update README.md

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In section

System Pre-requisites - chipmunk2d C library installation

subsection

Click to show instructions for ubuntu - method 1

code

sudo ln -s /home/rstudio/Chipmunk2D/include/chipmunk /usr/share/R/include/chipmunk
sudo ln -s /home/rstudio/Chipmunk2D/src/libchipmunk.so /usr/lib/R/lib/libchipmunk.so
sudo ln -s /home/rstudio/Chipmunk2D/src/libchipmunk.so.7 /usr/lib/R/lib/libchipmunk.so.7   # new line

After adding the new line, the installation no longer reports error cannot find *.so.7