Add FAQ with one question from chat

docs/modules/ROOT/nav.adoc

@@ -10,3 +10,4 @@
 * xref:examples.adoc[Examples]
 * xref:developer.adoc[Developer]
 ** xref:developer_stm32.adoc[Developer: STM32]
+* xref:faq.adoc[Frequently Asked Questions]

docs/modules/ROOT/pages/faq.adoc

@@ -0,0 +1,24 @@
+= Frequently Asked Questions
+
+These are a list of unsorted, commonly asked questions and answers.
+
+Please feel free to add items to link:https://github.com/embassy-rs/embassy/edit/main/docs/modules/ROOT/pages/faq.adoc[this page], especially if someone in the chat answered a question for you!
+
+== Missing main macro
+
+If you see an error like this:
+
+[source,rust]
+---
+#[embassy_executor::main]
+|                   ^^^^ could not find `main` in `embassy_executor`
+---
+
+You are likely missing some features of the `embassy-executor` crate.
+
+For Cortex-M targets, consider making sure that ALL of the following features are active in your `Cargo.toml` for the `embassy-executor` crate:
+
+* `arch-cortex-m`
+* `executor-thread`
+* `nightly`
+