add readme for data collection

src/data_collection/README.md

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+# Data Collection
+
+Data collection in bottom has two main components: **sources** and **collectors**.
+
+**Sources** are either libraries or system APIs that actually extract the data.
+These may map to multiple different operating systems. Examples are `sysinfo`,
+or `libc` bindings, or Linux-specific code.
+
+**Collectors** are _platform-specific_ (typically OS-specific), and can pull from
+different sources to get all the data needed, with some glue code in between. As
+such, sources should be written to be per-"job", and be divisible such that
+collectors can import specific code as needed.
+
+We can kinda visualize this with a quick-and-dirty diagram (note this is not accurate or up-to-date):
+
+```mermaid
+flowchart TB
+    subgraph sources
+        direction TB
+        linux
+        windows
+        macos
+        unix
+        sysinfo
+        freebsd
+    end
+    subgraph collectors
+        direction TB
+        Linux
+        Windows
+        macOS
+        FreeBSD
+    end
+    linux -..-> Linux
+    unix -..-> Linux
+    sysinfo -..-> Linux
+    windows -..-> Windows
+    sysinfo -..-> Windows
+    macos -..-> macOS
+    unix -..-> macOS
+    sysinfo -..-> macOS
+    freebsd -..-> FreeBSD
+    sysinfo -..-> FreeBSD
+```