add readme

README.md

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+# ft_ping
+## Why ?
+This project is a good way to understand how icmp and ip communication work (at a high level).
+After doing this project, I know how the unix ping command work
+
+## Compilation
+To compile this project, just execute the command `make` at the root of the project.
+
+## Usage
+```
+sudo ./ft_ping [options] <destination>...
+```
+
+example: 
+```
+sudo ./ft_ping -W 1 localhost -v google.fr -c 2
+```
+
+## limitations
+
+* needs super user right to run
+* only supports IPv4
+
+## bonus options
+
+* --ttl
+* --count (-c)
+* --linger (-W)
+* --timeout (-w)
+* --interval (-i)
+