README.md: Add more details about `fetch`
Make it clear right from the beginning that the fetching and the
building stages are separate by teaching the reader to run `fetch` first
and then `build`.
@@ -67,6 +67,14 @@ repository.
Performing a build
---
+First, we fetch all the metadata and packages:
+
+```
+$ coreos-assembler fetch
+```
+
+And now we can build from these inputs:
+
```
$ coreos-assembler build
```
@@ -76,7 +84,7 @@ as generate VM images in `/srv/coreos/builds/`.
Next, rerun `coreos-assembler build` and notice the system correctly
deduces that nothing changed. You can run `coreos-assembler fetch`
-to check for updated RPMs.
+again to check for updated RPMs.
Running
---
