feat(popcount): Clean up popularity counting script

Adds the script used to generate the popularity information for all of
nixpkgs.

The README lists the (currently somewhat rough) usage instructions.

1 files changed, 39 insertions, 0 deletions

diff --git a/tools/nixery/popcount/README.md b/tools/nixery/popcount/README.md
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+popcount
+========
+
+This script is used to count the popularity for each package in `nixpkgs`, by
+determining how many other packages depend on it.
+
+It skips over all packages that fail to build, are not cached or are unfree -
+but these omissions do not meaningfully affect the statistics.
+
+It currently does not evaluate nested attribute sets (such as
+`haskellPackages`).
+
+## Usage
+
+1. Generate a list of all top-level attributes in `nixpkgs`:
+
+   ```shell
+   nix eval '(with builtins; toJSON (attrNames (import <nixpkgs> {})))' | jq -r | jq > all-top-level.json
+   ```
+
+2. Run `./popcount > all-runtime-deps.txt`
+
+3. Collect and count the results with the following magic incantation:
+
+   ```shell
+   cat all-runtime-deps.txt \
+     | sed -r 's|/nix/store/[a-z0-9]+-||g' \
+     | sort \
+     | uniq -c \
+     | sort -n -r \
+     | awk '{ print "{\"" $2 "\":" $1 "}"}' \
+     | jq -c -s '. | add | with_entries(select(.value > 1))' \
+     > your-output-file
+   ```
+
+   In essence, this will trim Nix's store paths and hashes from the output,
+   count the occurences of each package and return the output as JSON. All
+   packages that have no references other than themselves are removed from the
+   output.