Create README.md (#60)

* Create README.md

add gtp5g version table

* Update README.md

add free5gc version

* Update README.md

add free5gc version

README.md

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+# go-upf
+### The specific version
+**Note:** Please make sure to check your UPF version and use a compatible gtp5g version according to the table below.
+
+|free5GC Version| UPF Version | Compatible gtp5g Versions |
+|-----------------|-----------------|--------------------------|
+|v3.4.3| v1.2.3 | >= 0.8.6 and < 0.9.0 |
+|v3.4.2| v1.2.3 | >= 0.8.6 and < 0.9.0 |
+|v3.4.1| v1.2.2 | >= 0.8.6 and < 0.9.0 |
+|v3.4.0| v1.2.1 | >= 0.8.6 and < 0.9.0 |
+|v3.3.0| v1.2.0 | >= 0.8.1 and < 0.9.0 |
+|v3.2.1| v1.1.0 | >= 0.7.0 and < 0.7.0 |
+|v3.2.0| v1.1.0 | >= 0.7.0 and < 0.7.0 |