[readme] add download/build instructions

README.md

@@ -3,6 +3,29 @@ Turbo NSS
 
 glibc nss library for passwd and group.
 
+Checking out and building
+-------------------------
+
+```
+$ git clone --recursive https://git.sr.ht/~motiejus/turbonss
+```
+
+Alternatively, if you forgot `--recursive`:
+
+```
+$ git submodule update --init
+```
+
+And run tests:
+
+```
+$ zig build test
+```
+
+... the other commands will be documented as they are implemented.
+
+This project uses [git subtrac][git-subtrac] for managing dependencies.
+
 Steps
 -----
 
@@ -72,3 +95,5 @@ The following operations need to be fast, in order of importance:
 4. lookup groupname -> group.
 5. (optional) iterate users using a defined order (`getent passwd`).
 6. (optional) iterate groups using a defined order (`getent group`).
+
+[git-subtrac]: https://github.com/apenwarr/git-subtrac/