Update readme for Reclient instead of Goma

With Goma being deprecated, setting up build configurations should use
Reclient instead for those that are allowed to use it.

Change-Id: I8b546242c552e150fa08920a5d343a77de207fff
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/115812
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Alan Screen <awscreen@chromium.org>

README.md

diff --git a/README.md b/README.md
index 9434470..d674025 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,7 @@
 A typical `<directory>` name is `out/Debug`.
 
 ```
-use_goma = false  # Googlers only. Ensure goma is installed and running first.
+use_remoteexec = false # Approved users only.  Do necessary setup & authentication first.
 is_debug = true  # Enable debugging features.
 
 # Set true to enable experimental Skia backend.