rotate string exam exercise readme

subjects/rostring.en.md

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+## rostring
+
+### Instructions
+
+Write a program that takes a string and displays this string after rotating it
+one word to the left.
+
+Thus, the first word becomes the last, and others stay in the same order.
+
+A "word" is defined as a part of a string delimited either by spaces/tabs, or
+by the start/end of the string.
+
+Words will be separated by only one space in the output.
+
+If the number of arguments is not one, the program displays \n.
+
+And its output :
+
+```console
+student@ubuntu:~/piscine/rostring$ go build
+student@ubuntu:~/piscine/rostring$ ./rostring "abc   " | cat -e
+abc$
+student@ubuntu:~/piscine/rostring$ ./rostring "Que la     lumiere soit et la lumiere fut"
+la lumiere soit et la lumiere fut Que
+student@ubuntu:~/piscine/rostring$ ./rostring "     AkjhZ zLKIJz , 23y"
+zLKIJz , 23y AkjhZ
+student@ubuntu:~/piscine/rostring$ ./rostring | cat -e
+$
+student@ubuntu:~/piscine/rostring$
+```