docs: describe tags in the table (#374)

Author: rvagg

README.md

@@ -46,6 +46,28 @@ Each multicodec is marked with a status:
 
 NOTE: Just because a codec is marked draft, don't assume that it can be re-assigned. Check to see if it ever gained wide adoption and, if so, mark it as permanent.
 
+### Tag Categories
+
+Each multicodec entry in the table.csv has a "tag" column that helps categorize the entry type. When adding a new multicodec, it's important to assign the correct tag. Here are descriptions of the most common tags:
+
+* **multihash**: Entries for cryptographic hash functions as described in [multihash](https://github.com/multiformats/multihash). Multihash is a protocol for differentiating outputs from various well-established cryptographic hash functions, addressing size and encoding considerations.
+
+* **hash**: Non-cryptographic hash functions, which are not suitable for content addressing systems but may have other uses. As noted in the [multihash documentation](https://github.com/multiformats/multihash#non-cryptographic-hash-functions), these have specialized use-cases where identifying non-cryptographic hash functions by multihash may be desirable.
+
+* **key**: Cryptographic key types, including public and private keys for various cryptographic algorithms.
+
+* **multiaddr**: Network address formats as defined in [multiaddr](https://github.com/multiformats/multiaddr). Multiaddr aims to make network addresses future-proof, composable, and efficient.
+
+* **ipld**: [InterPlanetary Linked Data](https://ipld.io/) formats. These codecs are suitable for use as a codec in a [CID](https://github.com/multiformats/cid) and will deserialize linked data that may contain links to other data.
+
+* **serialization**: Similar to "ipld" but does not materialize links. For example, JSON and CBOR have the "serialization" tag, while DAG-JSON and DAG-CBOR have the "ipld" tag because they handle links.
+
+* **varsig**: Signature formats as described in [varsig](https://github.com/ChainAgnostic/varsig). Varsig is a multiformat for describing signatures over IPLD data and raw bytes in a way that preserves information about the payload and canonicalization information.
+
+* **namespace**: Namespace identifiers for various protocols and systems.
+
+Selecting the appropriate tag helps maintain organization in the table and provides context for how a codec is expected to be used.
+
 ### Adding new multicodecs to the table
 
 The process to add a new multicodec to the table is the following: