Adding a Readme for IPv4CIDR package

ipv4cidr/README.md

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+# IPv4 CIDR Manager
+The package `IPv4CIDR` contains utilities to perform the following operations:
+
+1. Parse a string representing the CIDR block
+    - Take a single IP address as input
+    - Take a CIDR block in a standard notation where the `IP` part of the `IP/CIDR` range is the first IP address in the CIDR block
+    - Take a non-standard CIDR block and enable a `standardize` flag to convert it to the standard notation
+2. Split the CIDR block into two halves
+3. Get the following information from the CIDR block
+    - Convert to string
+    - Get the IP part of the block representation
+    - Get the CIDR mask part of the block representation
+    - Get the nth IP address in range
+    - Get the netmask
+    - Get the size of the CIDR block
+
+## To Use
+Import the package into your code using:
+
+    import "github.com/microsoft/go-cidr-manager/ipv4cidr