downloads: Add documentation on the PIN_TOOLCHAINS env var

Change-Id: I08dfaa9600817fc3849623b0bdbd2fe347a6d2ad

GettingStarted.md

diff --git a/GettingStarted.md b/GettingStarted.md
index 537e9a1..a38046e 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -155,6 +155,35 @@
 repo upload -t --re ${REVIEWER} --cc ${CC_LIST} # To upload the change to Gerrit for review
 ```
 
+### Help! I'm on a limited-bandwidth connection, and need to stop downloading tooling!
+
+There's an environment variable you can set called `PIN_TOOLCHAINS` to prevent
+download attempts for various toolchains in the system. The format is a
+space-delimited list of words that includes which toolchains you wish to prevent
+downloading automatically during a build.
+
+At the moment, this environment variable can be set to any of the following
+words:
+
+  `renode` - prevents downloading of the latest Renode binary build.
+
+  `iree`   - prevents downloading of the latest IREE compiler toolchain.
+
+Ie:
+
+```bash
+export PIN_TOOLCHAINS="renode iree"
+```
+
+*Note well*: if you set this variable, the output or behavior of any of the
+preceding toolchains may be bad or incorrect. Please do not file bugs against
+these components if you have this variable set!
+
+### Help! IREE, Renode, or another toolchain is misbehaving!
+
+Did you set `PIN_TOOLCHAINS`? If so, please unset this environment variable and
+retry your build.
+
 ## Repository Layout
 
 Our layout is pretty simple: