Add migration guide for v2.0.0

MigrationGuide.md

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+## coreSNTP version >=v2.0.0 Migration Guide
+
+With coreSNTP versions >=v2.0.0, there are breaking changes that need to be addressed when upgrading.
+
+### Breaking Changes
+
+* The `Sntp_ConvertToUnixTime` function now uses `uint32_t *` instead of `UnixTime_t *` for the Unix time seconds parameter. This change was made because:
+  - A `uint32_t` Unix timestamp can represent dates until February 7, 2106 06:28:15 UTC
+  - Beyond this date, we cannot differentiate between NTP era 0 and era 1 timestamps, thus it does not need to be a `uint64_t`
+
+Thus, the signature of `Sntp_ConvertToUnixTime` changed from:
+```c
+SntpStatus_t Sntp_ConvertToUnixTime( const SntpTimestamp_t * pSntpTime,
+                                      UnixTime_t * pUnixTimeSecs,
+                                      uint32_t * pUnixTimeMicrosecs );
+```
+
+to:
+
+```c
+SntpStatus_t Sntp_ConvertToUnixTime( const SntpTimestamp_t * pSntpTime,
+                                      uint32_t * pUnixTimeSecs,
+                                      uint32_t * pUnixTimeMicrosecs );
+```
+
+To migrate, update any code that uses `UnixTime_t` for storing the Unix time seconds value to use `uint32_t` instead.
+
+**Old Code Snippet**:
+```c
+SntpTimestamp_t sntpTime;
+UnixTime_t unixTimeSecs;
+uint32_t unixTimeMicrosecs;
+SntpStatus_t status;
+
+// Assume sntpTime has been populated from an SNTP response
+
+status = Sntp_ConvertToUnixTime( &sntpTime, &unixTimeSecs, &unixTimeMicrosecs );
+```
+
+**New Code Snippet**:
+```c
+SntpTimestamp_t sntpTime;
+uint32_t unixTimeSecs;
+uint32_t unixTimeMicrosecs;
+SntpStatus_t status;
+
+// Assume sntpTime has been populated from an SNTP response
+
+status = Sntp_ConvertToUnixTime( &sntpTime, &unixTimeSecs, &unixTimeMicrosecs );
+```