Add CI workflow for installcheck

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: make installcheck
+on: [push, pull_request]
+jobs:
+  test:
+    strategy:
+      matrix:
+        pg: [17, 16, 15, 14, 13, 12, 11, 10, 9.6]
+    name: PostgreSQL ${{ matrix.pg }}
+    runs-on: ubuntu-latest
+    container: pgxn/pgxn-tools
+    steps:
+
+      - name: Start PostgreSQL ${{ matrix.pg }}
+        run: pg-start ${{ matrix.pg }}
+
+      - name: Check out the repo
+        uses: actions/checkout@v2
+
+      - name: Test on PostgreSQL ${{ matrix.pg }}
+        run: pg-build-test

README.md

@@ -1,5 +1,8 @@
 COUNT_DISTINCT aggregate
 ========================
+
+[![make installcheck](https://github.com/tvondra/count_distinct/actions/workflows/ci.yml/badge.svg)](https://github.com/tvondra/count_distinct/actions/workflows/ci.yml)
+
 This extension provides an alternative to COUNT(DISTINCT ...) which for large
 amounts of data often ends in sorting and poor performance.