complete the README with intructions and a preview PDF

README.md

 # latex-poster-template
+A LaTeX template to write beautiful posters.
 
-A LaTeX template to write beautiful posters.
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+## Writing a new poster
+- fork this repository
+- put all you figures in the `figures/` directory
+
+## Compile the poster
+- make sure to install the `latexmk` package and command
+- run `make poster`
+
+> **Note**
+> the local repo can be cleaned from compilation files with `make clean`
+
+## The poster template
+![poster.pdf](poster.pdf)