docs: Split docs into four sections (#882)

Author: kyleconroy

docs/README.md

@@ -0,0 +1,67 @@
+# Developing sqlc
+
+## Building
+
+For local development, install `sqlc` under an alias. We suggest `sqlc-dev`.
+
+```
+go build -o ~/go/bin/sqlc-dev ./cmd/sqlc
+```
+
+## Running Tests
+
+```
+go test ./...
+```
+
+To run the tests in the examples folder, use the `examples` tag.
+
+```
+go test --tags=examples ./...
+```
+
+These tests require locally-running database instances. Run these databases
+using [Docker Compose](https://docs.docker.com/compose/).
+
+```
+docker-compose up -d
+```
+
+The tests use the following environment variables to connect to the
+database
+
+### For PostgreSQL
+
+```
+Variable     Default Value
+-------------------------
+PG_HOST      127.0.0.1
+PG_PORT      5432
+PG_USER      postgres
+PG_PASSWORD  mysecretpassword
+PG_DATABASE  dinotest
+```
+
+### For MySQL
+
+```
+Variable     Default Value
+-------------------------
+MYSQL_HOST      127.0.0.1
+MYSQL_PORT      3306
+MYSQL_USER      root
+MYSQL_ROOT_PASSWORD  mysecretpassword
+MYSQL_DATABASE  dinotest
+```
+
+## Regenerate expected test output
+
+If you need to update a large number of expected test output in the
+`internal/endtoend/testdata` directory, run the `regenerate.sh` script.
+
+```
+make regen
+```
+
+Note that this uses the `sqlc-dev` binary, not `sqlc` so make sure you have an
+up to date `sqlc-dev` binary.