Add a few missing README.md files

Change-Id: Idf9c70e5bf062150abfeae84c3fe0794062872bc

pw_checksum/README.md

diff --git a/pw_checksum/README.md b/pw_checksum/README.md
new file mode 100644
index 0000000..45d68e7
--- /dev/null
+++ b/pw_checksum/README.md
@@ -0,0 +1 @@
+# pw\_checksum: Checksum algorithms

pw_containers/README.md

diff --git a/pw_containers/README.md b/pw_containers/README.md
new file mode 100644
index 0000000..77e0b5e
--- /dev/null
+++ b/pw_containers/README.md
@@ -0,0 +1 @@
+# pw\_containers: Embedded-friendly container classes

pw_string/README.md

diff --git a/pw_string/README.md b/pw_string/README.md
index 2cfee8e..9c850c1 100644
--- a/pw_string/README.md
+++ b/pw_string/README.md
@@ -1 +1 @@
-# pw\_string: Utilities for building strings
+# pw\_string: Utilities for safely building strings

pw_tokenizer/README.md

diff --git a/pw_tokenizer/README.md b/pw_tokenizer/README.md
new file mode 100644
index 0000000..8bde14d
--- /dev/null
+++ b/pw_tokenizer/README.md
@@ -0,0 +1,3 @@
+# pw\_tokenizer: Tools for replacing string literals with binary tokens
+pw\_tokenizer is a portable, general purpose module for replacing printf-style
+string literals with compact binary tokens.