How can I use R to combine multiple rows of data into one row with a concatenated field?

Question

I'm new to R (just finished my Codecademy classes) and trying to apply it to a project I'm working on, but haven't had luck searching online. Could someone point me in the right direction here?

I have a .CSV formatted as:

| A header | B header | C header |
| -------- | -------------- || -------------- |
| First    | abc            || 001            |
| First    | abc            || 002            |
| First    | abc            || 003            |
| Second   | xyz            || 004            |
| Third    | abc            || 005            |
| Third    | abc            || 006            |

I'd like to merge rows where A Header and B Header match, and I'd like the resulting C Header to be a concatenated (eg, comma separated) field.

For example, my ideal result:

| A header | B header | C header |
| -------- | -------------- || -------------- |
| First    | abc            || 001, 002, 003  |
| Second   | xyz            || 004            |
| Third    | abc            || 005, 006       |

How would I go about this?

Thanks in advance.