Document BPF C program limitations

programs/bpf/c/sdk/README.md

@@ -21,7 +21,6 @@ $ sudo apt-get install -y clang-7
 ```
 
 ### macOS
-
 The following depends on Homebrew, instructions on how to install Homebrew are at https://brew.sh
 
 Once Homebrew is installed, ensure the latest llvm is installed:
@@ -31,3 +30,34 @@ $ brew install llvm  # <- should output “Warning: llvm 7.0.0 is already instal
 $ brew --prefix llvm # <- should output “/usr/local/opt/llvm”
 ```
 
+## 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) {
+  SolKeyedAccounts 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.
+
+### 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.