new README nbrconvertalpha for quest 06

subjects/nbrconvertalpha.en.md

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+## nbrconvertalpha
+
+### Instructions
+
+Write a **program** that prints the corresponding letter in the `n` position of the alphabet, where `n` is each argument received.
+
+For example 1 is a, 2 is b, etc. If `n` is not a valid position of the alphabet or if the argument is not an integer, the **program** should print a space (" ").
+
+It should be implemented a flag `--upper` that prints the result in upper case. 
+
+### Usage
+
+```console 
+student@ubuntu:~/student/test$ go build
+student@ubuntu:~/student/test$ ./nbrconvertalpha
+
+student@ubuntu:~/student/test$ ./nbrconvertalpha 8 5 12 12 15 | cat -e
+hello$
+student@ubuntu:~/student/test$ ./nbrconvertalpha 12 5 7 5 14 56 4 1 18 25 | cat -e
+legen dary$
+student@ubuntu:~/student/test$ ./nbrconvertalpha 32 86 h | cat -e
+   $
+student@ubuntu:~/student/test$ ./nbrconvertalpha --upper 8 5 25  | cat -e
+HEY$
+```