Re-org SDK dir (#4690)

sdk/bpf/c/README.md

@@ -0,0 +1,44 @@
+## Development
+
+### Quick start
+To get started create a `makefile` containing:
+```make
+include path/to/bpf.mk
+```
+and `src/program.c` containing:
+```c
+#include <solana_sdk.h>
+
+bool entrypoint(const uint8_t *input) {
+  SolKeyedAccount ka[1];
+  uint8_t *data;
+  uint64_t data_len;
+
+  if (!sol_deserialize(buf, ka, SOL_ARRAY_SIZE(ka), NULL, &data, &data_len)) {
+    return false;
+  }
+  print_params(1, ka, data, data_len);
+  return true;
+}
+```
+
+Then run `make` to build `out/program.o`.
+Run `make help` for more details.
+
+### Unit tests
+Built-in support for unit testing is provided by the
+[Criterion](https://criterion.readthedocs.io/en/master/index.html) test framework.
+To get started create the file `test/example.c` containing:
+```c
+#include <criterion/criterion.h>
+#include "../src/program.c"
+
+Test(test_suite_name, test_case_name) {
+  cr_assert(true);
+}
+```
+Then run `make test`.
+
+### Limitations
+* Programs must be fully contained within a single .c file
+* No libc is available but `solana_sdk.h` provides a minimal set of primitives