Update documentation

docs/guides/create-thread.md

@@ -1,5 +1,60 @@
 # Create a thread
 
-`Presence.SocialFormat.Lib` provides the `IThreadComposer` interface, allowing you to compose a thread from a collection of `Snippets`.
+`Presence.SocialFormat.Lib` provides a `ThreadBuilder` class, which can be used to build threads, based on a set of `SocialSnippet` inputs, for any number of different networks.
 
-**TODO: instructions**
+Each network has a number of rules, described with structs `ThreadCompositionRules`, `PostRenderRules`, and an optional `ICounterCreator` (the default counter is fairly sensible).
+
+Alternatively, you can reach the thread composers directly. Each implements the `IThreadComposer` interface, allowing you to compose a thread from a collection of `Snippets`.
+
+## Snippets
+
+A `SocialSnippet` is a piece of a post. `SnippetType` is an `enum` with several possible values, presenting the type of snippet:
+
+- `Text` - simple text (place the text in `.Text`)
+- `Link` - an external link (place link text in `.Text`, and the link url in `.Reference`)
+- `Tag` - a HashTag (place the tag in the `.Text` value)
+- `Break` - a special snippet to indicate that the thread composer must start a new post
+- `Counter` - a special text snippet created by the thread composer as a post number indicator
+
+Initialse a `SocialSnippet` with a simple constructor or the initialisation syntax:
+
+```csharp
+var snippet1 = new SocialSnippet("Simple text");
+var snippet2 = new SocialSnippet("This is a link", SnippetType.Link, "https://instantiator.dev");
+var snippet3 = new SocialSnippet()
+{
+    Text = "#Tagging",
+    SnippetType = SocialSnippetType.Tag
+};
+```
+
+## Building a thread
+
+Here's a the simplest possible example - add a single Message snippet (of `SnippetType.Text` by default), and build the thread.
+
+```csharp
+var posts = new ThreadBuilder(SocialNetwork.BlueSky)
+    .WithMessage(new SocialSnippet("Hello, world!"))
+    .Build();
+```
+
+The `ThreadBuilder.Build()` method returns an `IDictionary<IThreadComposer, IEnumerable<CommonPost>>` - a dictionary keyed by composers, with values of the thread that each composer created.
+
+You could add any number of additional snippets, together as an `IEnumerable<SocialSnippet>` or individually.
+
+Set tags for the thread, too - similarly, individually or together as an `IEnumerable<SocialSnippet>`:
+
+```csharp
+var posts = new ThreadBuilder(SocialNetwork.BlueSky)
+    .WithMessages(messageSnippets)
+    .WithTag(new SocialSnippet("#Example1", SnippetType.Tag))
+    .WithTag(new SocialSnippet("#Example2", SnippetType.Tag))
+    .Build();
+```
+
+By default, the `BlueSkyThreadComposer` will
+
+- limit text to `300` characters per post (including counters, hashtags)
+- render links as text rather than URLs (as BlueSky can specify link regions in posts)
+- apply hashtags to the first post only
+- apply a counter to the beginning of each post, if there is more than 1 post in the thread