docs: document L2CAP support and add a build workflow

.github/workflows/build.yml

@@ -0,0 +1,22 @@
+name: build
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v7
+
+      - uses: actions/setup-go@v7
+        with:
+          go-version: stable
+
+      - name: build
+        run: go build ./...
+
+      - name: vet
+        run: go vet ./...

README.md

@@ -20,6 +20,19 @@ Function and type names in cbgo are intended to match the corresponding CoreBlue
 * Named arguments in CoreBluetooth functions are eliminated.
 * Properties are implemented as a pair of functions (`PropertyName` and `SetPropertyName`).
 
+## L2CAP
+
+Connection-oriented channels are supported in both roles.  A central opens a
+channel to a peripheral and is handed the result through its peripheral
+delegate; a peripheral publishes a PSM and is handed incoming channels through
+its peripheral manager delegate.
+
+Reads and writes on an open channel never block.  They return a count of zero
+when the channel currently has no data to give or no space to take more, and
+report the end of the stream once the far end goes away.  Register an event
+handler on the channel to be told when either of those conditions changes,
+rather than polling for it.
+
 ## Issues
 
 There are definitely memory leaks.  ARC is not compatible with cgo, so objective C memory has to be managed manually.  I didn't see a set of consistent guidelines for object ownership in the CoreBluetooth documentation, so cbgo errs on the side of leaking.  Hopefully this is only an issue for very long running processes!  Any fixes here are much appreciated.