docs: a quick local dev setup and contributing sections

while there, made test filter actually work with

    zig build test -Dtest-filter=xxx

a previous invocation with `--test-filter` never worked.

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# zig fmt and clang-format all lib/ and src/
+698e5b6f76f4cec712e147e86f6c373f811e28d0

README.md

@@ -1,5 +1,62 @@
+# nakamochi daemon and gui (ndg)
+
 build for rpi:
 
     zig build -Dtarget=aarch64-linux-musl -Ddriver=fbev -Drelease-safe -Dstrip
 
 otherwise just `zig build` on dev host
+
+## local development
+
+you'll need [zig v0.10.x](https://ziglang.org/download/).
+if working on the gui, also [SDL2](https://www.libsdl.org/).
+
+note that compiling the daemon on macOS is currently unsupported since
+it requires some linux primitives.
+
+compiling is expected to be as easy as
+
+    # only gui
+    zig build ngui
+    # only daemon
+    zig build nd
+    # everything at once
+    zig build
+
+the output is placed in `./zig-out/bin` directory. for example, to run the gui,
+simply execute `./zig-out/bin/ngui`.
+
+run all tests with
+
+    zig build test
+
+or a filtered subset using `test-filter`:
+
+    zig build test -Dtest-filter=xxx
+
+significant contributors may find adding [.git-blame-ignore-revs](.git-blame-ignore-revs)
+file to their git config useful, to skip very likely irrelevant commits
+when browsing `git blame`:
+
+    git config blame.ignoreRevsFile .git-blame-ignore-revs
+
+see also the contributing section.
+
+## contributing
+
+to contribute, create a pull request or send a patch with
+[git send-mail](https://git-scm.com/docs/git-send-email) to alex-dot-cloudware.io.
+
+before sending a change, please make sure tests pass:
+
+    zig build test
+
+and all code is formatted: zig code with `zig fmt` and C according to the
+style described by [.clang-format](.clang-format) file. if `clang-format` tool
+is installed, all formatting can be checked with:
+
+    ./tools/fmt-check.sh
+
+note that only C files in `src/` are formatted.
+leave third party libraries as is - it is easier to update and upgrade when
+the original style is preserved, even if it doesn't match this project.

build.zig

@@ -102,14 +102,8 @@ pub fn build(b: *std.build.Builder) void {
         tests.setTarget(target);
         tests.setBuildMode(mode);
         tests.linkLibC();
-        if (b.args) |args| {
-            for (args) |a, i| {
-                if (std.mem.eql(u8, a, "--test-filter")) {
-                    tests.setFilter(args[i + 1]); // don't care about OOB
-                    break;
-                }
-            }
-        }
+        const f = b.option([]const u8, "test-filter", "run tests matching the filter");
+        tests.setFilter(f);
 
         const test_step = b.step("test", "run tests");
         test_step.dependOn(&tests.step);