Wikipedia: do in-place cobs_decode() in the encoded buffer (#29)

* do an in-place decode of wikipedia tests using cobs_decode() to prove we can

* update docs for in-place decoding

README.md

@@ -99,6 +99,10 @@ if (result == COBS_RET_SUCCESS) {
 
 ### Decoding In-Place
 
+There are two ways to decode COBS-encoded data in-place. One is simply by calling `cobs_decode` and pass your buffer to both the `enc` and `out_dec` parameters. Since COBS decoding never needs to revisit decoded bytes, and decoded payloads are always shorter than encoded payloads, this is guaranteed to work.
+
+`cobs_decode_inplace` is also provided and offers byte-layout-parity to `cobs_encode_inplace`. This lets you, for example, decode a payload, change some bytes, and re-encode it all in the same buffer:
+
 Accumulate data from your source until you encounter a COBS frame delimiter byte of `0x00`. Once you've got that, call `cobs_decode_inplace` on that region of a buffer to do an in-place decoding. The zeroth and zero bytes of your payload will be replaced with the `COBS_SENTINEL_VALUE` bytes that, were you _encoding_ in-place, you would have had to place there anyway.
 
 ```

tests/test_wikipedia.cc

@@ -43,6 +43,16 @@ void round_trip(byte_vec_t const &decoded, byte_vec_t const &encoded) {
                        &dec_actual_len) == COBS_RET_SUCCESS );
 
   REQUIRE( decoded == byte_vec_t(dec_actual, dec_actual + dec_actual_len) );
+
+  // Additionaly, in-place decode atop enc_actual using cobs_decode.
+
+  REQUIRE( cobs_decode(enc_actual,
+                       enc_actual_len,
+                       enc_actual,
+                       sizeof(enc_actual),
+                       &dec_actual_len) == COBS_RET_SUCCESS );
+
+  REQUIRE( decoded == byte_vec_t(enc_actual, enc_actual + dec_actual_len) );
 }
 }