expand string exam exercise readme

subjects/expandstr.md

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+## expandstr
+
+### Instructions
+
+Write a program that takes a string and displays it with exactly three spaces
+between each word, with no spaces or tabs either at the beginning or the end,
+followed by a newline.
+
+A word is a section of string delimited either by spaces/tabs, or by the
+start/end of the string.
+
+If the number of parameters is not 1, or if there are no words, simply display
+a newline.
+
+Example of output :
+
+```console
+student@ubuntu:~/student/expandstr$ go build
+student@ubuntu:~/student/expandstr$ ./expandstr "you   see   it's   easy   to   display   the   same   thing" | cat -e
+you   see   it's   easy   to   display   the   same   thing$
+student@ubuntu:~/student/expandstr$ ./expandstr "   only  it's harder   " | cat -e
+only   it's   harder$
+student@ubuntu:~/student/expandstr$ ./expandstr " how funny it is" "did you  hear, Mathilde ?" | cat -e
+$
+student@ubuntu:~/student/expandstr$ ./expandstr | cat -e
+$
+student@ubuntu:~/student/expandstr$
+```