Add spelling.md - a doc describing how to spell various noms objects (#1423)

* Add spelling.md - a doc describing how to spell datastores, datasets, and objects.

* review

doc/spelling.md

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+# Spelling in Noms
+
+Many commands and APIs in Noms accept datastore, dataset, or value specifications as arguments. This document describes how to construct these specifications.
+
+## Spelling Datastores
+
+Datastore specifications take the form:
+
+```
+<protocol>:<path>
+```
+
+The `path` part of the name is interpreted differently depending on the protocol:
+
+- **http(s):** specs describe a remote datastore to be accessed over HTTP. In this case, the entire datastore spec is a normal http(s) URL. For example: `https://dev.noms.io/aa`.
+- **ldb:** specs describe a local [LevelDB](https://github.com/google/leveldb)-backed datastore. In this case, the path component should be a relative or absolute path on disk to a directory in which to store the LevelDB data. For example: `ldb:~/noms-data`.
+- **mem:** specs describe an ephemeral memory-backed datastore. In this case, the path component is not used and must be empty.
+
+## Spelling Datasets
+
+Dataset specifications take the form:
+
+```
+<datastore>:<dataset>
+```
+
+See [spelling datastores](#spelling-datastores) for how to build the `datastore` part of the name. The `dataset` part is just any string matching the regex `^[a-zA-Z0-9\-_/]+$`. However, it is not advised to name datasets starting with `sha1-`.
+
+## Spelling Values
+
+Value specifications take the form:
+
+```
+<datastore>:<value-name>
+```
+
+See [spelling datastores](#spelling-datastores) for how to build the datastore part of the name.
+
+The `value-name` part can be either a ref or a dataset name. If  `value-name` matches the pattern `^sha1-[0-9a-fA-F]{40}$`, it will be interpreted as a ref. Otherwise it will be interpreted as a dataset name.
+
+### Examples
+
+```sh
+# “foo” dataset at http://api.noms.io/-/aa
+http://api.noms.io/-/aa:foo
+
+# value sha1-123 at http://localhost:8000
+http://localhost:8000/monkey:sha1-123
+
+# “bonk” dataset at ldb:/foo/bar
+ldb:/foo/bar:bonk
+```