Fix MD041 warnings

The following warnings are generated by using markdownlint analysis:
```
docs/README.md:1 MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## To Build"]
```
Refer to markdown-lint [1] to fix MD041
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/041-first-line-h1.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I2bdb36a03a1e3d454f30f69ebbb95b21cacd32f8
diff --git a/docs/README.md b/docs/README.md
index 6a3532a..398b2f1 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -1,3 +1,5 @@
+# phosphor-user-manager
+
 ## To Build
 
 To build this package, do the following steps: