Clearer docs for building

Summary: Per title.

Reviewed By: fabiomassimo

Differential Revision: D28284707

fbshipit-source-id: 60dde8653acd6bc05a570d622490f4632fc9f59a

scripts/bump/README.md

@@ -54,3 +54,43 @@ can be quite effective in reducing it:
 build:
   split-objs: true
 ```
+
+## Updating the binaries
+
+The platform-specific binaries in the parent directory (`bump.lnx64` and `bump.mac`)
+need to be manually updated after every change to the source here.
+
+To do this, first build the binary for Mac:
+
+```
+stack build
+cp .stack-work/install/x86_64-osx/<build_id>/8.6.5/bin/bump ../bump.mac
+```
+
+(The build ID is printed as part of the stack build run.)
+
+Then, add this to the options from above to produce static binaries on Linux
+(otherwise, there's a good chance Sandcastle won't run it). The binary section
+should then look like this:
+
+*package.yaml*
+```diff
+diff --git a/xplat/sonar/scripts/bump/package.yaml b/xplat/sonar/scripts/bump/package.yaml
+--- a/xplat/sonar/scripts/bump/package.yaml
++++ b/xplat/sonar/scripts/bump/package.yaml
+@@ -12,3 +12,4 @@
+   bump:
+     source-dirs:      .
+     main:             bump.hs
++    ghc-options:      -optl-static -optl-pthread -fPIC
+
+```
+
+With the docker daemon running, execute:
+
+```
+stack build --docker
+cp .stack-work/install/x86_64-linux-<image_id>/<build_id>/8.6.5/bin/bump ../bump.lnx64
+```
+
+Commit the changes, done.