Remove obsolete information from README.md (#1399)

Author: menelaos

README.md

@@ -1374,14 +1374,6 @@ workspace:
       - "arg2"
       - "arg3"
 
-  # Optional setting to enable the "lockfile". Enabling it will generate
-  # a `spago.lock` file with a cache of the build plan.
-  # It's disabled by default when using package sets (because we already
-  # get a stable build plan from there) and enabled by default when using
-  # the Registry solver.
-  # See "The lock file" section for more details.
-  lock: false
-
   # Optional section to further customise the build.
   buildOpts:
     # Directory for the compiler products - optional, defaults to `output`.
@@ -1587,11 +1579,10 @@ package:
 
 ### The lock file
 
-The lock file is a file that Spago can generate to cache the build plan, so that it can be reused in subsequent builds.
-
-When using package sets it is disabled by default - since we already get a stable build plan from there - while it's enabled by default when using the Registry solver.
-
-You can enable it manually by adding a `lock: true` field to the `workspace` section of your `spago.yaml` file, and that will keep it on regardless of which solving mode you're using.
+The lock file (`spago.lock`) is a file that is generated by Spago in order to
+ensure reproducible builds. It tracks the exact dependency tree of a package
+along with other metadata, such as integrity hashes, and should be committed to
+version control.
 
 ### File System Paths used in Spago